Add comprehensive unit tests for DQL tokenizer and highlighter

The plugin currently has no visible test suite. Adding tests would improve code reliability and make it easier to add new features without regressions.

**Proposed Changes:**
- Create unit tests for the DQL tokenizer (`nextToken` function)
- Add tests for edge cases (nested comments, escaped quotes, duration literals)
- Test the highlighting logic with various DQL syntax scenarios
- Add tests for command parsing and validation
- Set up CI/CD to run tests on pull requests

**Benefits:**
- Catch regressions early when adding new features
- Document expected behavior through tests
- Make it easier for contributors to add enhancements safely
- Improve code maintainability and refactoring confidence

**Related Code:**
Tests should cover `src/dql-highlight.ts` and command logic in `src/main.ts`. Consider using Jest or Vitest for testing.