Add php-parallel-lint to the PHP linting CI job

[GuySartorelli]

We use PHP Code Sniffer, but that’s not a linter. Notably, it won’t pick up on syntax errors.

We should add https://github.com/php-parallel-lint/PHP-Parallel-Lint to CI to help avoid syntax errors from sneaking in.

Notes

• We should probably also add this to the testing recipe so that it's included in installer and sink installations by default as a dev dependency, which facilitates running it locally.

Acceptance criteria

• [x] All supported modules have a php linting step that runs php-parallel-lint
• [x] The linter is run against all source code, tests, and _config.php if one exists

PRs

• https://github.com/silverstripe/gha-run-tests/pull/30
• https://github.com/silverstripe/recipe-testing/pull/22

[maxime-rainville]

If we had a syntax error, I would expect PHPUnit to start failing. I can't remember any time where our we had a syntax error that manage to sneak through this way.

Not necessarily against this, but it seems like a very marginal improvement.

[GuySartorelli]

It is absolutely a very marginal improvement. It's also very little work. IMO seems worth doing.