Fix moveBefore feature detection (#32087)

`moveBefore` was moved to the `ParentNode` mixin as per
https://github.com/whatwg/dom/pull/1307#discussion_r1881981120 _(and was
committed in
https://github.com/whatwg/dom/commit/3f3e94c5beda922962dacaeb606087f57bd7f7be)_

As a result, its existence can no longer be checked on `Node.prototype`
but must be checked in `Element.prototype`