process: fix experimental warning tracking

[Archkon]

Return Just(false) for duplicate experimental warnings and only record a warning after it has been emitted successfully.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.31%. Comparing base (c18fd90) to head (218ce26).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #65070   +/-   ##
=======================================
  Coverage   90.31%   90.31%           
=======================================
  Files         759      759           
  Lines      247646   247648    +2     
  Branches    46697    46694    -3     
=======================================
+ Hits       223654   223660    +6     
+ Misses      15463    15461    -2     
+ Partials     8529     8527    -2     

Files with missing lines	Coverage Δ
src/node_process-inl.h	100.00% <100.00%> (ø)
src/node_process_events.cc	73.13% <100.00%> (+13.43%)	⬆️

... and 27 files with indirect coverage changes

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