Build/Test Tools: Generate a code coverage report using GitHub Actions.
This introduces a new GitHub Action workflow to generate a code coverage report when running the PHPUnit test suite as both a single and multisite install.
The workflow will run once per week on Sunday at 00:00 UTC.
For now, the reports are not submitted anywhere, but they will be uploaded to the workflow run as ZIP file artifacts where they will persist for 90 days.
Making code coverage reports more readily available will hopefully better highlight areas of Core with poor coverage, and encourage more contributors to help increase test coverage of those areas.
Props ocean90, johnbillion.
Fixes #52034.