Parse common forms of character set as their intended node

[Jcparkyn]

[a-zA-Z] -> Wildcard set to 'letters' [0-9] -> Wildcard set to 'digits' (but this would create inconsistency with the parsed expression, so might be best avoided) Variant of [ \t\r\n] -> Whitespace with corresponding options ticked.

Inversions of these should also be parsed.