Character class `printable` in token regexes

[SchrodingerZhu]

Add printable to represent all printable unicode chars

[andreasabel]

Add printable to represent all printable unicode chars

Yes, this would be great to have, but I don't know how to implement this. printable would have to be mapped to the lexer generators our backends support, but I am not aware that e.g. Alex supports this, the issue https://github.com/simonmar/alex/issues/126 is still open.

See also #23.