This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.
One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.
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 :^)
Until now, we've been using CSS::LengthPercentage, sometimes wrapped in
Optional, to represent CSS sizes.
This meant we could not support modern values like `min-content`,
`max-content`, `fit-content(<length>)`. We were also conflating `none`
and `auto` which made the `min-*` and `max-*` properties confusing.
The new CSS::Size class covers all possible size values as individual
substates. It'll be quite a bit of work to make all layout code aware of
the additional features, this patch merely makes the new type available.
