Introduction of phpstan

[Chrico]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

This PR will replace "phpcs" with "phpstan" (level 8!) and adds additional extensions for phpstan/phpstan-deprecation-rules´ and swissspidy/phpstan-no-private`.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 4 lines in your changes missing coverage. Please review.

Project coverage is 98.23%. Comparing base (c79bb36) to head (7687fce).

Files with missing lines	Patch %	Lines
src/Container/ServiceExtensions.php	66.66%	2 Missing ⚠️
src/Properties/LibraryProperties.php	88.88%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #57      +/-   ##
============================================
- Coverage     98.84%   98.23%   -0.61%     
  Complexity      251      251              
============================================
  Files            10       10              
  Lines           607      624      +17     
============================================
+ Hits            600      613      +13     
- Misses            7       11       +4     

Flag	Coverage Δ
unittests	98.23% <88.88%> (-0.61%)	⬇️

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