Move code-quality checks into separate Github action tasks

[Art4]

I've noticed that sometimes the tests with PHP 8.3 fail because of issues with the upload to codecov. This is misleading, because seeing a failed test with PHP 8.3 I assume that there is something wrong with the code.

In this PR I've split the Github action tasks into tests (with PHPUnit) and code-quality checks (phpstan, code-coverage). I've also added a new check for code-sytle using php-cs-fixer.

This way we will see at first glance what's the problem with a broken task.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (712eddb) 96.71% compared to head (1617cc2) 96.71%.

Additional details and impacted files

@@            Coverage Diff            @@
##               v2.x     #358   +/-   ##
=========================================
  Coverage     96.71%   96.71%           
  Complexity      534      534           
=========================================
  Files            27       27           
  Lines          1459     1459           
=========================================
  Hits           1411     1411           
  Misses           48       48           

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[kbsali]

👍 I did not know about matrix (and it was already in place apparently 😅 )