Extract PHPStan into dedicated workflow

[jeromegamez]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 45.50%. Comparing base (7e1c682) to head (f27130f).
⚠️ Report is 2 commits behind head on 7.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                7.x    #1027      +/-   ##
============================================
+ Coverage     37.70%   45.50%   +7.80%     
  Complexity     1481     1481              
============================================
  Files           146      146              
  Lines          4347     4342       -5     
============================================
+ Hits           1639     1976     +337     
+ Misses         2708     2366     -342     

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