src: enter isolate before destructing IsolateData

MVP fix for a worker_threads crash where ~WorkerThreadData() ->
~IsolateData() -> Isolate::DetachCppHeap() kicks off a round of
garbage collection that expects an entered isolate.

No test because the crash is not reliably reproducable but the bug
is pretty clearly described in the linked issue and is obvious once
you see it, IMO.

Fixes: nodejs#51129

src/node_worker.cc

@@ -217,7 +217,13 @@ class WorkerThreadData {
       CHECK(!loop_init_failed_);
       bool platform_finished = false;
 
-      isolate_data_.reset();
+      // https://github.com/nodejs/node/issues/51129 - IsolateData destructor
+      // can kick off GC before teardown, so ensure the isolate is entered.
+      {
+        Locker locker(isolate);
+        Isolate::Scope isolate_scope(isolate);
+        isolate_data_.reset();
+      }
 
       w_->platform_->AddIsolateFinishedCallback(isolate, [](void* data) {
         *static_cast<bool*>(data) = true;