The shadowRoot property getter that will be added in subsequent commits
has an additional check that checks whether the shadow root is opened.
I didn't update the function logic to match with the IDL interface,
because it's very likely we don't want that check in the existing code,
so that for example closed shadow root elements can still be updated.
We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
