The old approach was basically a linear scan, which is slower than a
hash map for the currently 303 elements, as evidenced by the new
benchmark in TestCSSIDSpeed.
Before: Completed benchmark 'value_id_from_string' in 3238ms
After: Completed benchmark 'value_id_from_string' in 193ms
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.
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.