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/CSS/Screen.cpp

@@ -18,7 +18,7 @@ JS_DEFINE_ALLOCATOR(Screen);
 
 JS::NonnullGCPtr<Screen> Screen::create(HTML::Window& window)
 {
-    return window.heap().allocate<Screen>(window.realm(), window);
+    return window.realm().create<Screen>(window);
 }
 
 Screen::Screen(HTML::Window& window)