If the last child of a node is a Text child and we are inserting text in that node,
we need to append it to that Text child. Doing that means that
HTMLStyleElement::children_changed gets called less frequently, but that's not a
problem during parsing thanks to the pop hook.
Free-standing fonctions parse_html and friends are now static methods
on ServoParser, and the HTML and XML tokenizers have been moved to private
submodules.