Demix ChildNode

[Elchi3]

See #8929 for background.

IDL: https://dom.spec.whatwg.org/#interface-childnode

interface mixin ChildNode {
  [CEReactions, Unscopable] undefined before((Node or DOMString)... nodes);
  [CEReactions, Unscopable] undefined after((Node or DOMString)... nodes);
  [CEReactions, Unscopable] undefined replaceWith((Node or DOMString)... nodes);
  [CEReactions, Unscopable] undefined remove();
};
DocumentType includes ChildNode;
Element includes ChildNode;
CharacterData includes ChildNode;

So we need to expose compat data on DocumentType, Element, and CharacterData and remove the abstract compat data on ChildNode.

This PR assumes the same compat data on all specific interfaces.

[foolip]

I don't think any of these count as experimental at this point :)

[Elchi3]

Oh yes, I fixed this in content, too.