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/LibJS/Runtime/WeakMap.cpp

@@ -12,7 +12,7 @@ JS_DEFINE_ALLOCATOR(WeakMap);
 
 NonnullGCPtr<WeakMap> WeakMap::create(Realm& realm)
 {
-    return realm.heap().allocate<WeakMap>(realm, realm.intrinsics().weak_map_prototype());
+    return realm.create<WeakMap>(realm.intrinsics().weak_map_prototype());
 }
 
 WeakMap::WeakMap(Object& prototype)