src: simplify native immediate by using v8::Global

Unlike `node::Persistent`, `v8::Global` has move semantics and
can be used directly in STL containers.

PR-URL: #26254
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/env-inl.h

@@ -625,8 +625,7 @@ void Environment::CreateImmediate(native_immediate_callback cb,
   native_immediate_callbacks_.push_back({
     cb,
     data,
-    std::unique_ptr<Persistent<v8::Object>>(obj.IsEmpty() ?
-        nullptr : new Persistent<v8::Object>(isolate_, obj)),
+    v8::Global<v8::Object>(isolate_, obj),
     ref
   });
   immediate_info()->count_inc(1);

src/env.h

@@ -1139,7 +1139,7 @@ class Environment {
   struct NativeImmediateCallback {
     native_immediate_callback cb_;
     void* data_;
-    std::unique_ptr<Persistent<v8::Object>> keep_alive_;
+    v8::Global<v8::Object> keep_alive_;
     bool refed_;
   };
   std::vector<NativeImmediateCallback> native_immediate_callbacks_;