[USL] Support for raw strings in identifier content

[piotr-j]

Currently some characters are not allowed in content, namely ,, } and \n as they are used to find end of content in Lexer#lexIdentifierContent(). Optional " for raw strings would be nice.

Ive encountered this in definition for FreeTypeFontGenerator as it uses field for characters that should be generated.

While I dont really need this to be fixed, issue created in case someone else founds this problem.
One way this can be worked around is by using unicode escapes in the string instead of actual characters, ie \u007D instead of }

[kotcrab]

Thanks for reporting, might be worth adding it to the wiki.