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/MediaSourceExtensions/MediaSource.cpp

@@ -14,7 +14,7 @@ JS_DEFINE_ALLOCATOR(MediaSource);
 
 WebIDL::ExceptionOr<JS::NonnullGCPtr<MediaSource>> MediaSource::construct_impl(JS::Realm& realm)
 {
-    return realm.heap().allocate<MediaSource>(realm, realm);
+    return realm.create<MediaSource>(realm);
 }
 
 MediaSource::MediaSource(JS::Realm& realm)