feat: Add support for extension types, class type aliases and class modifiers

[spydon]

Description

Adds support for several modern Dart language features that the extractor did not handle. Previously, extension types and class type aliases would throw an AssertionError (Unknown declaration type: ...), and the Dart 3 class modifiers were silently dropped.

Extension types

extension type Meters(int value) implements num {} is now serialized with a new "extension-type" kind, including its representation declaration, const modifier, implements clause, type parameters and members:

{
  "kind": "extension-type",
  "name": "Meters",
  "representation": {"name": "value", "type": "int"},
  "implements": ["num"]
}

Class type aliases

class C = B with M implements I; (ClassTypeAlias) is now serialized as a "class" declaration with its extends/with/implements clauses and modifiers.

Class modifiers

The Dart 3 base, interface, final, sealed and mixin class modifiers are now reported as boolean fields on class declarations (and base on mixins), alongside the existing abstract field.

Notes

• The ExtensionTypeDeclaration and RepresentationDeclaration AST nodes are still flagged @experimental by the analyzer even though extension types have been stable since Dart 3.3, so a scoped experimental_member_use ignore is used.
• Following the repository convention, the version bump and CHANGELOG.md entry are left for a separate release PR.

Testing

• dart format .
• dart analyze --fatal-warnings --fatal-infos
• dart test (81 tests passing, up from 69)