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 :^)
This makes it possible to propagate exceptions from a function that
returns JS::ThrowCompletionOr via TRY() in another function that returns
WebIDL::ExceptionOr.
