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 namespace will be used for all interfaces defined in the URL
specification, like URL and URLSearchParams.
This has the unfortunate side-effect of requiring us to use the fully
qualified AK::URL name whenever we want to refer to the AK class, so
this commit also fixes all such references.
