ParentNode::$children returns a HTMLCollection of all directly
descendant child elements of a container.
I had to move around some properties such that the ParentNode property
offsets are always at a fixed offset, to simplify the code.
This also adds the necessary code to deal with GC cycles in
HTMLCollections.
Furthermore, we also disable cloning a HTMLCollection as that never
worked and furthermore it also conflicts with the [[SameObject]] WebIDL
requirement of $children.
GH-13516 was created before the new DOM files were added, and the PR was
never rebased to include the new DOM files, so there are three places
which were not replaced.
