watch: re-expose watch flags via execArgv

[Han5991]

Fixes: #60594

Summary

• Make the watch launcher capture the stripped --watch* flags and pass them to the child through NODE_WATCH_ARGS.
• During bootstrap, parse NODE_WATCH_ARGS, prepend the recovered flags to process.execArgv, and delete the env var.
• Extend test/sequential/test-watch-mode-watch-flags.mjs to assert that watch flags show up at the beginning of process.execArgv, accepting both --watch-path value and --watch-path=value forms.

Testing

• python3 tools/test.py sequential/test-watch-mode-watch-flags

[codecov]

Codecov Report

❌ Patch coverage is 81.25000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.74%. Comparing base (4f13746) to head (2447461).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/process/pre_execution.js	81.25%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60665      +/-   ##
==========================================
+ Coverage   89.72%   89.74%   +0.01%     
==========================================
  Files         675      675              
  Lines      204806   204822      +16     
  Branches    39355    39353       -2     
==========================================
+ Hits       183761   183813      +52     
+ Misses      13330    13280      -50     
- Partials     7715     7729      +14     

Files with missing lines	Coverage Δ
lib/internal/process/pre_execution.js	97.45% <81.25%> (+1.48%)	⬆️

... and 39 files with indirect coverage changes

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