Add support for extension type declarations

[Copilot]

Implements full support for Dart 3.3's extension type declarations as specified in the language specification.

Overview

Extension types are a compile-time abstraction mechanism introduced in Dart 3.3 that wraps an existing type with a different interface. This PR adds grammar rules to parse extension type declarations and their representation constructors.

Implementation

Added two new grammar rules to grammar.js:

extension_type_declaration - Implements the complete extension type syntax:

extension type const? <name> <typeParameters>? 
  <representationDeclaration> <interfaces>? 
  { <members>* }

representation_declaration - Handles the primary constructor with optional named/new prefix:

(.identifier | .new)? ( <metadata>? <type> <name> )

The implementation reuses existing rules for class_body, interfaces, and type_parameters to maintain consistency with the rest of the grammar.

Examples

All of these now parse correctly:

// Basic extension type
extension type IdNumber(int id) {
  operator <(IdNumber other) => id < other.id;
}

// With const modifier
extension type const UserId(int value) {}

// With type parameters
extension type Wrapper<T>(T value) {
  T unwrap() => value;
}

// With implements clause
extension type NumberE(int value) implements int {
  NumberE operator +(NumberE other) => NumberE(value + other.value);
}

// With named constructor
extension type PhoneNumber.fromInt(int number) {
  String format() => '($areaCode) $prefix-$lineNumber';
}

// With metadata
extension type UserId(@deprecated int id) {}

Testing

Added 7 comprehensive test cases covering:

• Basic extension types with operators
• const modifier
• Type parameters
• implements clause
• Named constructors (.identifier and .new)
• Metadata on representation parameters

Test Results:

• ✅ All 7 new extension type tests passing
• ✅ All 141 existing tests continue to pass
• ✅ No regressions introduced

Fixes #77

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Support for extension type declarations</issue_title>
<issue_description>The bodies of extension types seem to be handled gracefully, but the declaration isn't handled fully. They also support a limited version of the in-discussion primary constructor support.

Language docs: https://dart.dev/language/extension-types
Feature specification: https://github.com/dart-lang/language/blob/main/accepted/3.3/extension-types/feature-specification.md
Grammar:

<extensionTypeDeclaration> ::=
  'extension' 'type' 'const'? <typeIdentifier> <typeParameters>?
  <representationDeclaration> <interfaces>?
  '{'
    (<metadata> <extensionTypeMemberDeclaration>)*
  '}'

<representationDeclaration> ::=
  ('.' <identifierOrNew>)? '(' <metadata> <type> <identifier> ')'

<identifierOrNew> ::= <identifier> | 'new'

<extensionTypeMemberDeclaration> ::= <classMemberDefinition>
```</issue_description>

<agent_instructions>Based on the current tree-sitter grammar, do the minimally invasive edits to address the missing part of the grammar highlighted by this issue. Ensure that you add a few tests.</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

Fixes #82

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[TimWhiting]

Looks good to me. I've made some local changes for the nitpicks I had.

[TimWhiting]

Need to fix these test failures later.
@copilot Can you create an issue for fixing the CI?