add isAtomModifier to package exports

[jacekkopecky]

When dealing with key binding matching events, it is useful to be able to ignore failed bindings for modifiers, e.g. in atom/vim-mode#781.
This PR adds isAtomModifier to the exports of KeymapManager.

[nathansobo]

I'm reluctant to expand the API here. How much of a hardship would it be to just test the keystroke yourself with a regex?

[jacekkopecky]

It will be no hardship at all if you promise never to change the regex. 😀
Would it be possible to export just the regex somehow?

[nathansobo]

I really don't foresee the format of the binding sequences changing, and that should be the only thing that matters.

[jacekkopecky]

It's really this line that could foreseeably be expanded:

AtomModifiers.add(modifier) for modifier in ['ctrl', 'alt', 'shift', 'cmd']

But I can take your word for it on this one; and send the maintainer @maxbrunsfeld this way if he complains about a PR copying private code from one package into another. 😎

[nathansobo]

Yeah, I get that it's annoying to not always have an API for stuff. I just feel like APIs get gross as we add all these random things. In this case a bit of duplication for a pretty special case is okay with me. The set of modifiers should be stable. Famous last words.

[jacekkopecky]

Fair enough, I'll close this now.