Added core rule "flags"

[forman]

Added core rule "flags".

Checklist (strike out non-applicable):

• Changes documented in CHANGES.md
• Related issue exists and is referred to in the PR description and CHANGES.md
• Added docstrings and API docs for any new/modified user-facing classes and functions
• Changes/features documented in docs/*
• Unit-tests adapted/added for changes/features
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.24%. Comparing base (222e33f) to head (1432546).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
+ Coverage   97.15%   97.24%   +0.08%     
==========================================
  Files          52       53       +1     
  Lines        2108     2175      +67     
  Branches      270      286      +16     
==========================================
+ Hits         2048     2115      +67     
  Misses         32       32              
  Partials       28       28              

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