Single level per check

[reuvenharrison]

This PR splits checks that have multiple severity levels into multiple checks, each with a single level.
This will allow us to:

• make severity levels customizable (see Configurable selection of checks and their severity levels #572)
• correct oasdiff checks output
• continue making the checks model more structured which will improve their coverage and accuracy (see Improve breaking checks accuracy #580)

[codecov]

Codecov Report

Attention: Patch coverage is 96.12903% with 6 lines in your changes missing coverage. Please review.

Project coverage is 85.92%. Comparing base (ea17e6d) to head (886cc2a).
Report is 3 commits behind head on main.

Files	Patch %	Lines
checker/rules.go	92.59%	4 Missing ⚠️
checker/config.go	87.50%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #579      +/-   ##
==========================================
+ Coverage   85.89%   85.92%   +0.02%     
==========================================
  Files         228      227       -1     
  Lines       12974    13027      +53     
==========================================
+ Hits        11144    11193      +49     
- Misses       1415     1420       +5     
+ Partials      415      414       -1     

Flag	Coverage Δ
unittests	85.92% <96.12%> (+0.02%)	⬆️

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