Faster CLDR lookup for po

[vorlif]

Summary

• Documentation extended
• Faster conversion of the CLDR category to the gettext plural form.
• Constants for file extensions marked as deprecated, as they offer no additional value.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2c8406b) 87.79% compared to head (3e94e53) 87.81%.
Report is 1 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
+ Coverage   87.79%   87.81%   +0.02%     
==========================================
  Files          43       43              
  Lines        4063     4070       +7     
==========================================
+ Hits         3567     3574       +7     
  Misses        353      353              
  Partials      143      143              

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