Fix highlighting of long Unicode escape sequences

The CSS specification states that hexadecimal escape sequences up to six digits long are allowed within both single- and double-quoted strings. Previously, double-quoted strings allowed up to six decimal digits, but not hexadecimal. And double-quoted strings didn't allow more than one character escape sequences.

This change unifies the pattern used for both single- and double-quoted strings. It also allows both string types to have escape sequences consisting of a backslash followed by any character or a backslash followed by one to six hexadecimal digits.

/cc @kevinsawicki