chore: configure checkstyle

[vil02]

This PR adds checkstyle to this project. The checkstyle.xml is derived from sun_checks.xml by commenting out some some of the rules - they are marked by TODO. I think it might be a good idea to introduce some of them in the future. I will create an issue, where one can discuss which rules should be enabled.

The build.yml will fail if any warning would be found.

At the moment there is some unused import in one of the tests. Fixed in #5113.

@siriak what is your opinion?

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 37.23%. Comparing base (089b1f7) to head (6db59cc).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5110   +/-   ##
=========================================
  Coverage     37.23%   37.23%           
  Complexity     2343     2343           
=========================================
  Files           517      517           
  Lines         15630    15630           
  Branches       2975     2975           
=========================================
  Hits           5820     5820           
  Misses         9522     9522           
  Partials        288      288           

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[siriak]

Looks good, thanks!