watch: strip underscore aliases of watch flags

[trivikr]

Fixes: #66364

The C++ option parser treats _ and - as the same character in option names, so --watch_path is the same as --watch-path.

Watch mode compared spellings literally when it removed its own flags from process.execArgv and NODE_OPTIONS before spawning the child. The underscore spellings got through, so each child went back into watch mode and spawned another child, and the application never ran.

Normalize option names before comparing them. Forward the original arguments to the child unchanged.

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Assisted-by: claude:opus-5.5

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.35%. Comparing base (75e4bbe) to head (3ef07c5).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66365      +/-   ##
==========================================
- Coverage   90.36%   90.35%   -0.02%     
==========================================
  Files         792      792              
  Lines      275398   275424      +26     
  Branches    52776    52779       +3     
==========================================
- Hits       248877   248861      -16     
- Misses      16937    16982      +45     
+ Partials     9584     9581       -3     

see 32 files with indirect coverage changes

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