don't omit 'VAR' keyword when prettyprinting

The presence of `VAR` was originally stored in `flags.optional`, then ignored.

These changes create `flags.var` and use it in the parser and in exppp.

[Reported](https://github.com/stepcode/stepcode/pull/286#issuecomment-35847567) by @TRThurman, discovered by @craigl64