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/WebAudio/OscillatorNode.cpp

@@ -26,7 +26,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<OscillatorNode>> OscillatorNode::construct_
 {
     // FIXME: Invoke "Initialize the AudioNode" steps.
     TRY(verify_valid_type(realm, options.type));
-    auto node = realm.vm().heap().allocate<OscillatorNode>(realm, realm, context, options);
+    auto node = realm.create<OscillatorNode>(realm, context, options);
     return node;
 }