Clarify regexp support

[wavebeem]

Some flags for regexps cannot be reasonably supported within Parsimmon, so let's make it an error to use them!

Only i, u, and m make sense to support https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp.

Thanks to everyone in #81 for the discussion that brought up this issue!

I think it might make sense to just put this check in the assertRegexp function.

[wavebeem]

Also, the documentation should explicitly state that all regexps are wrapped in ^(?: ◊ ), and we should clarify which regexp flags are supported.