Each of these strings would previously rely on StringView's char const*
constructor overload, which would call __builtin_strlen on the string.
Since we now have operator ""sv, we can replace these with much simpler
versions. This opens the door to being able to remove
StringView(char const*).
No functional changes.
The `CSSMediaRule::serialized()` code is to spec. The
`CSSSupportsRule::serialized()` code has no spec right now, but I'm
fairly confident it will be almost identical to media's, so I copied
that for now.
The main thing missing is that we don't serialize the supports clause,
but for actually using a `@supports (something: cool) {}` rule in CSS,
it works!
