child_process: add subprocess.timedOut

[marcopiraccini]

When a child process receives its kill signal because timeout expired, callers cannot distinguish that from a signal sent for another reason.

Add subprocess.timedOut for spawn(), exec(), execFile() and fork(), and error.timedOut on errors returned by exec() and execFile(). The flag becomes true only when the timeout successfully sends the signal; it does not guarantee that the process has terminated.

This revisits #51608, which proposed the same property for #51561 but closed without landing. That review requested documentation and noted failing CI. This PR includes API documentation and regression tests for timeout expiry, normal exit, manual termination, non-timeout errors, and descendants keeping stdio open after the child exits.

Fixes: #51561
Refs: #51608

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.43%. Comparing base (ae5a0f4) to head (3989ee0).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66639      +/-   ##
==========================================
- Coverage   92.78%   90.43%   -2.35%     
==========================================
  Files         422      791     +369     
  Lines      193692   276601   +82909     
  Branches    29881    53116   +23235     
==========================================
+ Hits       179718   250148   +70430     
- Misses      13645    16872    +3227     
- Partials      329     9581    +9252     

Files with missing lines	Coverage Δ
lib/child_process.js	91.44% <100.00%> (+0.04%)	⬆️
lib/internal/child_process.js	95.13% <100.00%> (+1.68%)	⬆️

... and 500 files with indirect coverage changes

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