parser: Reject TypeScript-only syntax in JavaScript export declarations

[Copilot]

This PR fixes an issue where TypeScript-only syntax like export enum a {} was not being rejected when parsing JavaScript files (lang=js).

Problem

The parser correctly rejected standalone TypeScript syntax in JavaScript files:

enum a {} // ✅ Correctly errors with "Unexpected token"

However, when the same syntax was exported, it was incorrectly accepted:

export enum a {} // ❌ Was incorrectly parsed successfully
export type B = {}; // ❌ Was incorrectly parsed successfully  
export interface C {}; // ❌ Was incorrectly parsed successfully

Root Cause

The issue occurred in the export declaration parsing path. When parsing export <declaration>, the code calls parse_declaration in ts/statement.rs, which directly parsed TypeScript-only constructs without checking if the parser was in TypeScript mode (is_ts).

Solution

Added TypeScript mode checks in parse_declaration for the following constructs:

• enum declarations → TS8003: "'enum' can only be used in TypeScript files."
• type alias declarations → TS8004: "'type' alias can only be used in TypeScript files."
• interface declarations → TS8005: "'interface' can only be used in TypeScript files."
• module declarations → TS8006: "'module' can only be used in TypeScript files."
• namespace declarations → TS8007: "'namespace' can only be used in TypeScript files."

The parser still constructs a complete AST (for tooling compatibility) but emits appropriate TypeScript-specific diagnostic messages when these constructs are used in JavaScript files.

Verification

After the fix:

// JavaScript files now properly error:
export enum a {} // ❌ TS(8003): 'enum' can only be used in TypeScript files.
export type B = {}; // ❌ TS(8004): 'type' alias can only be used in TypeScript files.
export interface C {}; // ❌ TS(8005): 'interface' can only be used in TypeScript files.

// TypeScript files continue to work:
export enum a {} // ✅ Parses successfully
export type B = {}; // ✅ Parses successfully  
export interface C {}; // ✅ Parses successfully

Fixes #13205.

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[graphite-app]

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[Boshen]

hmmm I should git ignore but cache this in CI.