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/Streams/TransformStream.cpp

@@ -23,7 +23,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<TransformStream>> TransformStream::construc
 {
     auto& vm = realm.vm();
 
-    auto stream = realm.heap().allocate<TransformStream>(realm, realm);
+    auto stream = realm.create<TransformStream>(realm);
 
     // 1. If transformer is missing, set it to null.
     auto transformer = transformer_object.has_value() ? JS::Value { transformer_object.value() } : JS::js_null();