I think it would be really valuable for the project to set up built-in code coverage reporting for both PHPUnit and Behat tests in all the repositories.

For plugins shipping with WP-CLI commands I have added code coverage before, so for WP-CLI itself it should be doable too.

This will touch a lot of places:

1. Coming up with default configuration we can add to https://github.com/wp-cli/wp-cli-tests and to all the individual repos (e.g. codecov.yml file)
2. Update https://github.com/wp-cli/scaffold-package-command to add code coverage badges
3. Update https://github.com/wp-cli/.github to add an extra step for collecting and reporting code coverage
4. Add Codecov app to the whole organization to allow uploading

This is especially important when working on the upload logic in #6004, but will also be helpful beyond that.