[test] Add code coverage framework

Implement a comprehensive test coverage framework that integrates with pytest-cov
to generate code coverage reports for the yt-dlp codebase. Key components:

- Configuration file (.coveragerc) with appropriate include/exclude patterns
- Helper script (run_coverage.py) with parallel report generation
- GitHub Actions workflow for automatic coverage reporting on PRs and pushes
- Support for Hatch testing environment and CLI integration
- Testing documentation for running coverage reports
- Sample test for demonstrating coverage reporting

.gitignore

@@ -127,3 +127,6 @@ yt-dlp.zip
 # Plugins
 ytdlp_plugins/
 yt-dlp-plugins
+
+# Coverage
+/.coverage-reports/