worker: emit worker exit notifications on BroadcastChannel

[SudhansuBandha]

Expose worker termination notifications through BroadcastChannel so consumers can observe when a worker exits and inspect its thread ID and exit code.

Fixes: #59053

[Labeeb2339]

The current lint-cpp check reports 10 style errors in the new C++ code, so this PR is not yet lint-clean. Notably: trailing whitespace at src/node_messaging.cc:855 and src/node_messaging.h:194/233/235, an over-80-character line at src/node_messaging.cc:896, missing spaces in if(environment->is_stopping()) at :970, and incorrect indentation around WorkerExitNotification in src/node_messaging.h:226-229. Running the repository C++ formatter/linter and committing its output should address these before the functional review proceeds.

[codecov]

Codecov Report

❌ Patch coverage is 98.57143% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.06%. Comparing base (9f37c77) to head (4dd297f).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_messaging.cc	98.43%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #65575   +/-   ##
=======================================
  Coverage   90.05%   90.06%           
=======================================
  Files         751      751           
  Lines      254868   254937   +69     
  Branches    48107    48116    +9     
=======================================
+ Hits       229531   229608   +77     
+ Misses      16511    16495   -16     
- Partials     8826     8834    +8     

Files with missing lines	Coverage Δ
lib/internal/worker/io.js	99.20% <100.00%> (+<0.01%)	⬆️
src/node_messaging.h	63.15% <ø> (ø)
src/node_messaging.cc	83.59% <98.43%> (+0.55%)	⬆️

... and 40 files with indirect coverage changes

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