The Codecov service is a way to monitor test vs code coverage of a project over time and allows for the code coverage % + delta to be reported in each PR.
This commits:
* Adds a Codecov configuration.
* Adds a convenience script to the `composer.json` file to run the tests with or without code coverage.
* Adds a new matrix variable to the GH Actions `test` workflow to run the tests with code coverage and send the results to the Codecov service.
Notes:
- This disables the code coverage reporting in the "normal" test runs, including disabling `xdebug` for those runs which should make them slightly faster.
- This splits the test runs into two sets:
* High/low PHP are being run with code coverage (and have been removed from the "normal" test run matrix).
* For all other PHP versions, the tests are being run without code coverage.
* Adds a badge to the README to show the current code coverage %.
jrfnl