Fix V8 N-API leaking every promise ever created

[bkaradzic-microsoft]

Problem

v8impl::Persistent<T> was aliased to v8::Persistent<T>, whose default traits set
kResetInDestructor = false — the destructor does not dispose the global handle.

ConcludeDeferred() (behind napi_resolve_deferred / napi_reject_deferred) relies on
that destructor:

v8impl::Persistent<v8::Value>* deferred_ref = NodePersistentFromJsDeferred(deferred);
...
delete deferred_ref;

So the delete frees the wrapper but leaks the handle, pinning every promise created via
napi_create_promise — and everything its settled value retains — for the life of the
isolate. napi_env__::context_persistent leaks the same way. v8impl::Reference is
unaffected; it calls persistent_.Reset() explicitly.

Fix

Alias v8::Global<T>, which always resets in its destructor. This is what upstream
Node.js uses in
src/js_native_api_v8_internals.h;
the divergence came from vendoring, where the original node::Persistent<T> (a
v8::Persistent with kResetInDestructor = true) was swapped for the plain V8 type.

v8::Global is move-only like the non-copyable v8::Persistent it replaces, and every
other member used (Reset, SetWeak, ClearWeak, IsEmpty, Local::New) lives on
v8::PersistentBase, so no call sites change.

Testing

Static finding from the source, the V8 headers, and upstream parity; no measured
before/after attached. To confirm at runtime, run a long-lived host that settles many
promises (e.g. an image decode returning one promise per image) and watch external
memory / global handle count. Happy to add that before merging.

[Copilot]

Pull request overview

This PR fixes a V8 N-API memory leak where N-API-created promises (and related persistent handles) could be pinned for the lifetime of the isolate due to v8::Persistent not resetting in its destructor under the default traits.

Changes:

• Switch v8impl::Persistent<T> from v8::Persistent<T> to v8::Global<T> so persistent handles are released on destruction.
• Aligns this internal N-API V8 handle type with upstream Node.js behavior.

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[bghgary]

[Reviewed by Copilot on behalf of @bghgary]

This was my mistake from the original fork, not later drift — the file was hand-written to drop Node's internal dependencies, and the alias came out v8::Persistent.

Fixing it properly means syncing this file from upstream rather than patching it, which is separate work. This change is fine on its own.