Use "falls through" for all fallthrough comments

[mstange]

Main | Deploy preview

I was experimenting with oxlint, and oxlint's check needs the verbatim "falls through" spelling (unless we want to customize the pattern).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.11%. Comparing base (52baf1a) to head (a39bd15).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #6344    +/-   ##
========================================
  Coverage   84.11%   84.11%            
========================================
  Files         356      356            
  Lines       38444    38444            
  Branches    10786    10887   +101     
========================================
  Hits        32339    32339            
  Misses       5676     5676            
  Partials      429      429            

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