src: keep main-thread Isolate attached to platform during Dispose

This works around a situation in which the V8 WASM code calls
into the platform while the Isolate is being disposed.

This goes against the V8 API constract for `v8::Platform`.
In lieu of a proper fix, it should be okay to keep the Isolate
registered; the race condition fixed by 25447d8 cannot
occur for the `NodeMainInstance`’s Isolate, as it is the last
one to exit in any given Node.js process.

This partially reverts 25447d8.

Refs: nodejs#30909
Refs: nodejs#31752

PR-URL: nodejs#31795
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/node_main_instance.cc

@@ -102,8 +102,12 @@ NodeMainInstance::~NodeMainInstance() {
   if (!owns_isolate_) {
     return;
   }
-  platform_->UnregisterIsolate(isolate_);
+  // TODO(addaleax): Reverse the order of these calls. The fact that we first
+  // dispose the Isolate is a temporary workaround for
+  // https://github.com/nodejs/node/issues/31752 -- V8 should not be posting
+  // platform tasks during Dispose(), but it does in some WASM edge cases.
   isolate_->Dispose();
+  platform_->UnregisterIsolate(isolate_);
 }
 
 int NodeMainInstance::Run() {