Add repeatMatch functions to DSL

[natecook1000]

These allow you to spell literal expressions like a{3,} or a{2,5} when using the DSL. I've gone with the repeatMatch name to align a bit with repeatElement in the stdlib, and because repeating(...) sounds too much to my ear like you have to repeat the matched text, not repeat the match, if that makes sense.

Usage:

let regex = Regex {
    repeatMatch(2...) {
        repeatMatch(CharacterClass.word, count: 3)
        CharacterClass.digit
    }
}
"abc1".matches(regex)        // false
"abc1def2".matches(regex)    // true
"abc1defg2".matches(regex)   // false

[milseman]

LGTM, I'll let @rxwei comment on generator script.

My vote is for

repeat {
  ...
}

repeat(3...) {
  ...
}

I think repeat as a verb is fine alongside capture, but I'm not a picky grammar namer.

[rxwei]

LGTM. I'd vote for repeat as the name.

[rxwei]

Nit: How about a static method DSLTree.Node.repeating(_:_:_:)

[natecook1000]

I think repeat as a verb is fine alongside capture, but I'm not a picky grammar namer.

repeat is out, unfortunately, since it's a keyword, and people would need to write e.g. `repeat`(3...) { ... }. I've gone with repeating for now.

[natecook1000]

@swift-ci Please test