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/CanvasPattern.cpp

@@ -141,7 +141,7 @@ WebIDL::ExceptionOr<JS::GCPtr<CanvasPattern>> CanvasPattern::create(JS::Realm& r
     // FIXME: 7. If image is not origin-clean, then mark pattern as not origin-clean.
 
     // 8. Return pattern.
-    return realm.heap().allocate<CanvasPattern>(realm, realm, *pattern);
+    return realm.create<CanvasPattern>(realm, *pattern);
 }
 
 void CanvasPattern::initialize(JS::Realm& realm)