CI/QA: Switch to GitHub Actions and start using Coveralls

[jrfnl]

👉🏻 In most repo's which have a similar setup, I normally have "quicktest" and a "test" workflow, where "quicktest" has a smaller matrix and is run on push events and the "test" workflow has the full matrix and is run on pull events (and releases).
This is done based on those repos having a develop and a stable branch. As this repo doesn't have that, I've not modularized the builds that way here.

👉🏻 You can see the results of these workflows on the "Actions tab".

👉🏻 I'd recommend disconnecting the CircleCI integration in the Settings as otherwise the builds will always still show as failed (due to CircleCI not finding a configuration).

👉🏻 The Coveralls configuration may need some tweaking. Suggested settings:

CI/QA: Remove CircleCI configuration

... and all work-arounds in place to accommodate. Hope I caught them all.

Composer.json - widen PHPUnit requirements

PHP 5.4 needs PHPUnit 4.x to be able to run the tests, so that needed adding.
Aside from that PHPUnit 9.x has come out quite a while back and has better support for PHP 8.0 and 8.1 than PHPUnit 8.x.

I've set the version constraints for the 4.x and 5.x series to very specific minimum versions for a reason: those are the first versions to have the forward-compatibility layer with the namespaced PHPUnit TestCase, which is used by the tests in this repo.

CI/QA: add GH Actions workflows

This commit adds two workflows which run the same checks as were run in CircleCI + a lot more.

CS and QA workflow: checkcs job

Differences between this workflow and these checks when run via CircleCI:

• New: This workflow will now also run QA checks on the ruleset.xml file.
  It will run two checks:
  1. Does the ruleset validate against the schema ?
  2. Does the ruleset have a consistent code style, including consistent indentation using four spaces ?
• Removed: CircleCI ran PHPCS against both PHP 7.3 as well as 7.4. This is redundant as PHPCS should give the same results cross-version. In the new setup, it will only be run once against PHP 7.4.
• Changed: The PHPCS check will now be run using PHPCS master as an early warning system for upstream bugs.
• New: By using the cs2pr tool and the xmllint-problem-matcher, violations found will be shown in the GH code views, like the "Files" tab on a PR.

CS and QA workflow: phpstan job

Differences between this workflow and these checks when run via CircleCI:

• Removed: CircleCI ran PHPStan against both PHP 7.3 as well as 7.4. This is redundant as PHPStan, same like PHPCS, should give the same results cross-version. In the new setup, it will only be run once against PHP 7.4.
• New: PHPStan natively supports showing violations found in the GH code views, like the "Files" tab on a PR.

Test workflow: test job

• Changed: CircleCI only ran the tests against PHP 5.4, 5.6, 7.3 and 7.4 and only against the latest version of PHPCS.
  In this new setup, the tests are run against all supported PHP versions + the high/low supported PHPCS, which at this time means: PHPCS 3.5.6 as "low" and PHPCS dev-master as "high".
  This results in a lot more test runs, but this will safeguard the cross-version compatibility of the standard much better as well (and GH Actions is fast, so it shouldn't really impact CI runtime anyway).
• New: Tests are now also run against PHP 8.0, 8.1 and 8.2, where the run against PHP 8.2 is still allowed to fail for now (continue-on-error).

GH Actions/Test: add code coverage test runs & send to Coveralls

This commits:

• Splits the test runs into two jobs, where code coverage is run against the high/low PHP/PHPCS combinations and the "normal" test runs remain for all other PHP/PHPCS combination.
• Includes setting up cache warming for PHPUnit 9.3+ as PHP-Parser can interfere otherwise by backfilling tokens when they are not (yet) available in PHPCS.
• Adjusts the PHPUnit configuration to allow for measuring code coverage.
• Adds a new script to the composer.json file for running the tests with code coverage and adjusts the existing test script to ignore code coverage.
• Adds the badge from Coveralls to the README.

[sirbrillig]

Looks good! Thank you!