Fix prompt-fiddle highlighting (#2082)

This was generated mostly with claude-code.

 - Add a lezer grammar test suite
 - Fix the lezer grammar
 - Add grammar rules for expression functions
 - Allow comments in expr fns

Before: 
<img width="517" alt="Screenshot 2025-06-27 at 9 57 44 PM"
src="https://github.com/user-attachments/assets/65f62318-6ecc-46cd-a580-65e1b6496fae"
/>

After: 
<img width="531" alt="Screenshot 2025-06-27 at 11 07 33 PM"
src="https://github.com/user-attachments/assets/7609503e-d831-45cd-96ad-4d8f4a107f54"
/>

<!-- ELLIPSIS_HIDDEN -->


----

> [!IMPORTANT]
> Fixes BAML grammar issues and adds comprehensive test suite for
improved language support.
> 
>   - **Grammar Fixes**:
> - Fix `expr_block` in `datamodel.pest` to allow comments and empty
lines in expression functions.
> - Update `stmt` rule to allow trailing comments and optional newlines.
>     - Add grammar rules for expression functions in `syntax.grammar`.
>   - **Testing**:
> - Add a lezer grammar test suite in `test/test.cjs` and
`test/test.js`.
> - Add test cases for class, enum, function, and expression function
declarations in `test/cases/`.
>   - **Misc**:
> - Update `package.json` to change test script to use `test/test.cjs`.
> 
> <sup>This description was created by </sup>[<img alt="Ellipsis"
src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=BoundaryML%2Fbaml&utm_source=github&utm_medium=referral)<sup>
for df01047. You can
[customize](https://app.ellipsis.dev/BoundaryML/settings/summaries) this
summary. It will automatically update as commits are pushed.</sup>


<!-- ELLIPSIS_HIDDEN -->

Author: imalsogreg

engine/baml-lib/ast/src/parser/datamodel.pest

@@ -233,11 +233,11 @@ top_level_assignment = { stmt }
 expr_fn = { ("function" | "fn") ~ identifier ~ named_argument_list ~ ARROW? ~ field_type_chain? ~ expr_block }
 
 // Body of a function (including curly brackets).
-expr_block = { BLOCK_OPEN ~ NEWLINE? ~ (stmt ~ NEWLINE)* ~ expression ~ NEWLINE? ~ BLOCK_CLOSE }
+expr_block = { BLOCK_OPEN ~ NEWLINE? ~ (stmt | comment_block | empty_lines)* ~ expression ~ NEWLINE? ~ BLOCK_CLOSE }
 
 // Statement.
 // Currently the only statement is a let-binding.
-stmt = { let_expr ~ SEMICOLON? }
+stmt = { let_expr ~ SEMICOLON? ~ trailing_comment? ~ NEWLINE? }
 
 // Let-binding statement.
 let_expr = { "let" ~ identifier ~ "=" ~ expression }

engine/baml-lib/ast/src/parser/parse_expr.rs

@@ -146,6 +146,14 @@ pub fn parse_expr_block(token: Pair<'_>, diagnostics: &mut Diagnostics) -> Optio
             Rule::NEWLINE => {
                 continue;
             }
+            Rule::comment_block => {
+                // Skip comments in function bodies
+                continue;
+            }
+            Rule::empty_lines => {
+                // Skip empty lines in function bodies
+                continue;
+            }
             _ => {
                 diagnostics.push_error(DatamodelError::new_static(
                     "Internal Error: Parser only allows statements and expressions in function body.",

engine/baml-lib/baml/tests/validation_files/expr/builtin.baml

@@ -21,6 +21,18 @@ function GetTodoMissingTypeArg() -> Todo {
   })
 }
 
+// warning: Workflow functions are experimental, and will break in the future.
+//   -->  expr/builtin.baml:8
+//    | 
+//  7 | 
+//  8 | function GetTodo() -> Todo {
+//    | 
+// warning: Workflow functions are experimental, and will break in the future.
+//   -->  expr/builtin.baml:16
+//    | 
+// 15 | 
+// 16 | function GetTodoMissingTypeArg() -> Todo {
+//    | 
 // error: Generic function std::fetch_value must have a type argument. Try adding a type argument like this: std::fetch_value<Type>
 //   -->  expr/builtin.baml:17
 //    | 

engine/baml-lib/baml/tests/validation_files/expr/constructors.baml

@@ -19,4 +19,27 @@ class Person {
   name string
   age int
   poem string
-}
+}
+
+// warning: Variable assignment is experimental, and will break in the future.
+//   -->  expr/constructors.baml:6
+//    | 
+//  5 | 
+//  6 | let x = MyClass { a: 1, b: "2" };
+//    | 
+// warning: Variable assignment is experimental, and will break in the future.
+//   -->  expr/constructors.baml:8
+//    | 
+//  7 | 
+//  8 | let y = MyClass { a: 1, ..x };
+//    | 
+// warning: Variable assignment is experimental, and will break in the future.
+//   -->  expr/constructors.baml:11
+//    | 
+// 10 | 
+// 11 | let default_person = Person {
+// 12 |     name: "John Doe",
+// 13 |     age: 20,
+// 14 |     poem: "Never was there a man more plain."
+// 15 | };
+//    | 

engine/baml-lib/baml/tests/validation_files/expr/constructors_invalid.baml

@@ -18,6 +18,12 @@ fn Foo() -> Bar {
   x
 }
 
+// warning: Workflow functions are experimental, and will break in the future.
+//   -->  expr/constructors_invalid.baml:6
+//    | 
+//  5 | 
+//  6 | fn Foo() -> Bar {
+//    | 
 // error: Error validating: Bar.a expected type int, but found string
 //   -->  expr/constructors_invalid.baml:7
 //    | 

engine/baml-lib/baml/tests/validation_files/expr/constructors_nested.baml

@@ -8,6 +8,12 @@ class Bar {
 
 let x = Foo { bar: Bar { name: 10 }};
 
+// warning: Variable assignment is experimental, and will break in the future.
+//   -->  expr/constructors_nested.baml:9
+//    | 
+//  8 | 
+//  9 | let x = Foo { bar: Bar { name: 10 }};
+//    | 
 // error: Error validating: Bar.name expected type string, but found int
 //   -->  expr/constructors_nested.baml:9
 //    | 

engine/baml-lib/baml/tests/validation_files/expr/expr_fn.baml

@@ -4,6 +4,7 @@ function Foo(x: int) -> int {
 }
 
 fn Bar(x: int) -> int {
+    // test
     let y = x;
     x
 }
@@ -13,4 +14,17 @@ test TestBar {
     args {
         x 1
     }
-}
+}
+
+// warning: Workflow functions are experimental, and will break in the future.
+//   -->  expr/expr_fn.baml:1
+//    | 
+//    | 
+//  1 | function Foo(x: int) -> int {
+//    | 
+// warning: Workflow functions are experimental, and will break in the future.
+//   -->  expr/expr_fn.baml:6
+//    | 
+//  5 | 
+//  6 | fn Bar(x: int) -> int {
+//    | 

engine/baml-lib/baml/tests/validation_files/expr/expr_full.baml

@@ -1,4 +1,3 @@
-
 function MakePoem(length: int) -> string {
     client GPT4o
     prompt #"Write a poem {{ length }} lines long."#
@@ -68,4 +67,39 @@ client<llm> GPT4o {
 test TestMakePoem() {
     functions [MakePoem]
     args { length 4 }
-}
+}
+
+// warning: Workflow functions are experimental, and will break in the future.
+//   -->  expr/expr_full.baml:27
+//    | 
+// 26 | 
+// 27 | function Pipeline() -> string {
+//    | 
+// warning: Workflow functions are experimental, and will break in the future.
+//   -->  expr/expr_full.baml:37
+//    | 
+// 36 | 
+// 37 | function Pyramid() -> string {
+//    | 
+// warning: Workflow functions are experimental, and will break in the future.
+//   -->  expr/expr_full.baml:41
+//    | 
+// 40 | 
+// 41 | function OuterPyramid() -> string {
+//    | 
+// warning: Variable assignment is experimental, and will break in the future.
+//   -->  expr/expr_full.baml:19
+//    | 
+// 18 | 
+// 19 | let poem = MakePoem(10);
+//    | 
+// warning: Variable assignment is experimental, and will break in the future.
+//   -->  expr/expr_full.baml:21
+//    | 
+// 20 | 
+// 21 | let another = {
+// 22 |   let x = MakePoem(10);
+// 23 |   let y = MakePoem(5);
+// 24 |   CombinePoems(x,y)
+// 25 | };
+//    | 

engine/baml-lib/baml/tests/validation_files/expr/expr_list.baml

@@ -4,4 +4,17 @@ function Id(x: int) -> int {
 
 function Go(x: int) -> int[] {
  [x, Id(x), 3]
-}
+}
+
+// warning: Workflow functions are experimental, and will break in the future.
+//   -->  expr/expr_list.baml:1
+//    | 
+//    | 
+//  1 | function Id(x: int) -> int {
+//    | 
+// warning: Workflow functions are experimental, and will break in the future.
+//   -->  expr/expr_list.baml:5
+//    | 
+//  4 | 
+//  5 | function Go(x: int) -> int[] {
+//    | 

engine/baml-lib/baml/tests/validation_files/expr/expr_small.baml

@@ -1,3 +1,10 @@
 function A(x: int) -> int {
     x
-}
+}
+
+// warning: Workflow functions are experimental, and will break in the future.
+//   -->  expr/expr_small.baml:1
+//    | 
+//    | 
+//  1 | function A(x: int) -> int {
+//    |