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 is `<general-enclosed>` in CSS grammar. It represents a section of
a `@media` or `@supports` rule that exists in some future standard that
we don't understand yet, but don't want to make the entire rule invalid.
There's not much that it can usefully do, but we store a string
representation of its contents so that it can be serialized out.
