These don't have to worry about the input not being valid UTF-8 and
so can be infallible (and can even return self if no changes needed.)
We use this instead of Infra::to_ascii_{upper,lower}_case in LibWeb.
(cherry picked from commit 073bcfd3866852a4c4bcca2bd131bd65ae53541f)
While DeprecatedString and StringView use ASCII case-insensitivity when
matching, String uses the Unicode rules, so in order to match the spec,
we need to *not* use `String::equals_ignoring_case()`.
This function needs to be used everywhere that the spec refers to
an "ASCII case-insensitive match".
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 :^)
