feat(gen): add fallback handler to handlers.tmpl

[rglKali]

This way we can set up some "default" or "fallback" behaviour for the "unmatched" update types. Extremely useful for Entities caching, or just see all "skipped" by default updates

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.49%. Comparing base (c7f741b) to head (afbaaff).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1636      +/-   ##
==========================================
- Coverage   70.57%   70.49%   -0.09%     
==========================================
  Files         443      443              
  Lines       17945    17945              
==========================================
- Hits        12665    12650      -15     
- Misses       4341     4353      +12     
- Partials      939      942       +3     

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