Add mutation `/(foo)/` → `/(?>foo)/`

[backus]

[dgollahon]

@mbj I'm undecided on how useful I think this mutation is. We already mutate from /(foo)/ to /(?:foo)/ so we already get the lack of capture that this would achieve. I think atomic groups probably only make sense in fairly niche situations so I'm worried it may be confusing/misleading to generate atomic group mutations when we already have the mutation to non-capturing. I'm assuming we also don't want to go from atomic -> non-capturing or non-capturing -> atomic, but wanted to get your opinion here.

I have an implementation locally (it is extremely simple) but I am waiting to push until there is a chance for discussion.