Add String#match!

[devnote-dev]

This would be a non-nilable version of String#match. A lot of the standard library has methods for something that can error and something that should return nil instead, for example, the #[] and #[]? convention which is used throughout the standard library. This method does not, and requires doing one of the following for usage:

match = "Crystal".match(/(cr)/i).not_nil![1]

"Crystal".match(/(cr)/i)
match = $1

While you could use $1 to get around this, it becomes a pain to manage when you have a large number of capture groups or a recursive capture group, and it's not easy to identify which $ variable represents what capture. Ideally, String#match? would be preferable here, but String#match can return nil and to change that to non-nilable would be a breaking change. It's a slight inconsistency, but it's better than nothing.