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.
The code was really messy with lots of checks and inconsistencies.
This splits everything up into different functions and now everything is
relayed to a handler vtable.
