We already have a FlyString of its value from parsing, and most users
also want a FlyString from it, so let's use that instead of converting
backwards and forwards.
The two users that did want a String are:
- Quotes, which make sense as FlyString instead, so I've converted that.
- Animation names, which should probably be FlyString too, but the code
currently also allows for other kinds of StyleValue, and I don't want
to dive into this right now to figure out if that's needed or not.
In order to access the string's contents, use the new
`StringStyleValue::string_value()` method.
I think I found all the existing places that relied on
`StringStyleValue::to_string()` returning an unquoted string, but it's
hard to know for sure until things break.
