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/HTML/ServiceWorkerContainer.cpp

@@ -40,7 +40,7 @@ void ServiceWorkerContainer::visit_edges(Cell::Visitor& visitor)
 
 JS::NonnullGCPtr<ServiceWorkerContainer> ServiceWorkerContainer::create(JS::Realm& realm)
 {
-    return realm.heap().allocate<ServiceWorkerContainer>(realm, realm);
+    return realm.create<ServiceWorkerContainer>(realm);
 }
 
 // https://w3c.github.io/ServiceWorker/#navigator-service-worker-register