Move newline escaping in `CSSString` into `escape()`

[oliverklee]

#1444 (comment)
https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Values/string#syntax

[JakeQZ]

Also from https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Values/string

To output new lines, you must escape them with a line feed character such as \A or \00000A.