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 88.53%. Comparing base (4f1ab8c) to head (3e2a96f).
⚠️ Report is 1 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   88.03%   88.53%   +0.50%     
==========================================
  Files         703      703              
  Lines      208257   208273      +16     
  Branches    40069    40150      +81     
==========================================
+ Hits       183336   184402    +1066     
+ Misses      16883    15880    -1003     
+ Partials     8038     7991      -47     

Files with missing lines	Coverage Δ
lib/internal/process/pre_execution.js	98.56% <81.25%> (+3.75%)	⬆️

... and 97 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.