Properly type the return value of _languageExtForPath

[canova]

Apparently this was made this way since the typescript migration. I don't see a reason to keep it this way. Typing it works well.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.45%. Comparing base (c886717) to head (23f3ffb).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5937   +/-   ##
=======================================
  Coverage   85.45%   85.45%           
=======================================
  Files         321      321           
  Lines       32139    32139           
  Branches     8850     8850           
=======================================
  Hits        27463    27463           
  Misses       4241     4241           
  Partials      435      435           

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