Migrate to TypeScript

[jcfranco]

Description
Update the countries-and-timezones codebase to TypeScript for improved type safety, better developer experience, and easier maintenance.

Tasks

• Convert existing .js files to .ts
• Add type definitions for internal modules and public APIs
• Set up tsconfig.json and update build tooling as needed
• Ensure compatibility with current consumers (e.g., CommonJS/ESM support)
• Verify existing tests pass after migration

This should be a non-breaking change for consumers relying on the current public API.