Add `CustomConsumingRegexComponent

[DandyLyons]

Swift Regex allows you to use and/or implement your own custom regular expression behavior using CustomConsumingRegexComponent.

We should examples of implementing and using this.

[DandyLyons]

A new RegexCustomParsers namespace has been added to collect examples of implementing and using custom regular expressions using CustomConsumingRegexComponent.

[DandyLyons]

It seems that CustomConsumingRegexComponent is not up to feature parity with other RegexComponents. (I haven't confirmed this yet.)

I've filed a bug with the Swift team here: swiftlang/swift-experimental-string-processing#765