Nullary operators are indistinguishable from unary operators

[kymckay]

As a result of my decision to tokenise SQF commands as a single type of token and spot misuse errors semantically instead of syntactically (due to parsing complication that would require since commands can have all/some of nullary, unary and binary forms) the parser is currently unable to distinguish nullary from unary commands (since they both start with a keyword token).

I need to add a way for the parser to see ahead to the next token in order to handle this.