src: remove experimental ShadowRealm support

[anonrig]

This removes experimental ShadowRealm support from Node.js.

The TC39 ShadowRealm proposal remains unimplemented by browsers. WPT dropped ShadowRealm test coverage in web-platform-tests/wpt#59794 because the tests added significant noise without implementer support.

This PR:

• Removes the --experimental-shadow-realm flag and the --harmony-shadow-realm V8 option alias
• Removes the Node.js host integration (node_shadow_realm.*, isolate callback, bootstrap)
• Removes Node.js ShadowRealm tests and dedicated WPT fixtures that WPT already deleted
• Leaves V8's own ShadowRealm implementation untouched

--experimental-shadow-realm was never enabled by default and never left experimental status.

Refs: web-platform-tests/wpt#59794
Refs: #42528

This change was prepared with assistance from an AI coding agent.

[nodejs-github-bot]

Review requested:

• @nodejs/config
• @nodejs/gyp
• @nodejs/loaders
• @nodejs/realm
• @nodejs/startup
• @nodejs/web-standards

[jasnell]

While the status is still unclear, and it's not certain at all that'll it will make progress, it is still considered an active TC-39 proposal. Removing entirely may be premature. Won't block but I also don't agree with removing it just yet.

[legendecas]

Technically the proposal was moved to stage 2.7, and not moved yet stage 3 back again for implementation. We could add it back when it gets promoted to stage 3 again.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.33%. Comparing base (c0ed28f) to head (eff3e1a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65650      +/-   ##
==========================================
+ Coverage   90.29%   90.33%   +0.04%     
==========================================
  Files         790      787       -3     
  Lines      272883   272731     -152     
  Branches    52118    52079      -39     
==========================================
- Hits       246387   246377      -10     
+ Misses      16943    16841     -102     
+ Partials     9553     9513      -40     

Files with missing lines	Coverage Δ
lib/internal/bootstrap/realm.js	96.95% <100.00%> (-0.05%)	⬇️
lib/internal/errors.js	98.96% <100.00%> (+0.14%)	⬆️
lib/internal/process/pre_execution.js	96.47% <ø> (+0.71%)	⬆️
src/api/environment.cc	79.16% <ø> (-0.01%)	⬇️
src/async_wrap.cc	83.28% <ø> (ø)
src/env.cc	81.86% <ø> (-0.23%)	⬇️
src/env.h	97.26% <ø> (ø)
src/node_context_data.h	85.71% <ø> (ø)
src/node_options.cc	80.76% <ø> (-0.10%)	⬇️
src/node_options.h	95.61% <ø> (-0.02%)	⬇️
... and 1 more

... and 41 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/77001/

[mcollina]

We might want to wait until the next TC39 (I think it's later this month), and if nothing is moving still, we should remove it unless we get some alternative guidance from them.

[anonrig]

Rebased onto current main (eff3e1ad456). The only conflict was src/env.cc: kept the new node_profiling.h include, and left node_shadow_realm.h out. V8's own ShadowRealm code is untouched. What's gone is the Node host hook, --experimental-shadow-realm, and the --harmony-shadow-realm alias.

@legendecas agreed, we can put it back if this reaches stage 3 again. It's 2.7 right now.

@mcollina the next meeting is 29 Sep–1 Oct in Tokyo. This push is just to get the branch current. Not trying to land it ahead of that.

@jasnell I get why this feels early while the proposal is still active. Leaving it open so we can decide after the meeting.