neg method
Returns a parser that consumes any input token (character), but the receiver.
For example, the parser letter().neg() accepts any input but a letter.
The parser fails for inputs like 'a' or 'Z', but succeeds for
input like '1', '_' or '$'.
Implementation
@useResult
Parser<String> neg({String message = 'input not expected'}) =>
any().skip(before: not(message: message));