Harden dictionary cache reading (avoid Function constructor)

Summary

Replace the current approach that evaluates cache files using the Function constructor with a safer, simpler JSON-based format and reader. Provide a seamless migration path from the existing export default {...}; format to JSON, and add tests to cover read/write and migration scenarios.

Motivation

• Using new Function(...) to evaluate a cache file is unnecessarily risky and harder to maintain.
• A plain JSON cache improves safety, tooling compatibility, and clarity.
• Migration can be implemented without breaking users by auto-detecting and converting the old format upon first write.

Current behavior

• File: packages/compiler/src/lib/lcp/cache.ts
• The cache writer writes export default { ... }; and the reader strips that header and evaluates the remainder with new Function(...).
• This requires executing code to read the cache and is not ideal.

Proposed changes

1. Change cache file format to JSON

   • Write cache as JSON to a .json file (for example, lingo-cache.json), or keep the current filename but ensure pure JSON content.
   • Read cache with JSON.parse.

2. Backward-compatible migration

   • Detect the legacy format that starts with export default.
   • Parse legacy content by converting it to an object without executing code (e.g., using a minimal, safe transform and JSON.parse if possible, or a strict parser), then immediately re-write the cache in the new JSON format.
   • Consider logging a one-time info message when migration occurs (guarded by a debug flag).

3. Tests

   • Add tests that cover:
     • Reading/writing the new JSON cache format.
     • Migrating from an existing legacy export default {...}; cache file to the new JSON format.
     • Ensuring no data loss and deterministic formatting (stable key ordering already exists in writer code).

Affected files (initial)

• packages/compiler/src/lib/lcp/cache.ts (read/write and format logic)
• Potentially references to LCP_DICTIONARY_FILE_NAME if the extension changes (packages/compiler/src/_const)
• New tests near packages/compiler/src/lib/lcp/ (or existing test suite location)

Can I start working on it? Maintainers [@vrcprl @maxprilutskiy @sumitsaurabh927 @davidturnbull ..]