interpreter: (ematch) add get_or_create_class

[jumerckx]

This is a helper function that is used when doing class unions (#5708)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.35%. Comparing base (74f9dfe) to head (0856290).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5709      +/-   ##
==========================================
+ Coverage   86.33%   86.35%   +0.01%     
==========================================
  Files         406      406              
  Lines       57686    57774      +88     
  Branches     6702     6713      +11     
==========================================
+ Hits        49806    49890      +84     
- Misses       6322     6325       +3     
- Partials     1558     1559       +1     

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