LibJS+LibWeb: Use realm.create<T> instead of heap.allocate<T>

The main motivation behind this is to remove JS specifics of the Realm
from the implementation of the Heap.

As a side effect of this change, this is a bit nicer to read than the
previous approach, and in my opinion, also makes it a little more clear
that this method is specific to a JavaScript Realm.

Libraries/LibWeb/DOM/NodeIterator.cpp

@@ -54,7 +54,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<NodeIterator>> NodeIterator::create(Node& r
     // 2. Set iterator’s root and iterator’s reference to root.
     // 3. Set iterator’s pointer before reference to true.
     auto& realm = root.realm();
-    auto iterator = realm.heap().allocate<NodeIterator>(realm, root);
+    auto iterator = realm.create<NodeIterator>(root);
 
     // 4. Set iterator’s whatToShow to whatToShow.
     iterator->m_what_to_show = what_to_show;