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 :^)
These are from the HTML spec and therefore belong in the HTML/ directory
in LibWeb. Bindings/ has become a bit of a dumping ground, so this is a
first step towards cleaning that up.