Add support for completion of extends and is (#3443)

fixes #3152

.chronus/changes/ide-support-completion-for-extends-is-2024-4-24-11-15-1.md

@@ -0,0 +1,27 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/compiler"
+---
+
+Support completion for keyword 'extends' and 'is'
+
+  Example
+  ```tsp
+  model Dog ┆ {}
+            | [extends]
+            | [is]
+  
+  scalar Addresss ┆ 
+                  | [extends]
+
+  op jump ┆ 
+          | [is]
+  
+  interface ResourceA ┆ {}
+                      | [extends]
+
+  model Cat<T ┆> {}
+              | [extends]
+  ```
+

packages/compiler/src/core/charcode.ts

@@ -247,17 +247,20 @@ export function isNonAsciiIdentifierCharacter(codePoint: number) {
   return lookupInNonAsciiMap(codePoint, nonAsciiIdentifierMap);
 }
 
-export function codePointBefore(text: string, pos: number): number | undefined {
-  if (pos <= 0 || pos >= text.length) {
-    return undefined;
+export function codePointBefore(
+  text: string,
+  pos: number
+): { char: number | undefined; size: number } {
+  if (pos <= 0 || pos > text.length) {
+    return { char: undefined, size: 0 };
   }
 
   const ch = text.charCodeAt(pos - 1);
   if (!isLowSurrogate(ch) || !isHighSurrogate(text.charCodeAt(pos - 2))) {
-    return ch;
+    return { char: ch, size: 1 };
   }
 
-  return text.codePointAt(pos - 2);
+  return { char: text.codePointAt(pos - 2), size: 2 };
 }
 
 function lookupInNonAsciiMap(codePoint: number, map: readonly number[]) {