[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

devscripts/cov-combine

@@ -0,0 +1,5 @@
+#!/bin/sh
+# This script is a helper for the Hatch test coverage command
+# It's called by `hatch test --cover`
+
+coverage combine "$@"