Implement DOMTokenList.{add,remove,toggle}

Specs:
https://dom.spec.whatwg.org/#dom-domtokenlist-add
https://dom.spec.whatwg.org/#dom-domtokenlist-remove
https://dom.spec.whatwg.org/#dom-domtokenlist-toggle
https://dom.spec.whatwg.org/#concept-dtl-update
https://dom.spec.whatwg.org/#concept-ordered-set-serializer

Closes #3138.