Type-erasing `Regex<AnyRegexOutput>.init(_:)` is not implemented

[christopherweems]

Throws a fatal error with FIXME: Not implemented

I'm raising this issue as request to not drop this functionality (however unlikely that is).
It will have its uses!

[christopherweems]

https://github.com/apple/swift-experimental-string-processing/blob/88dc9dd3f10e6e49a1aae39113e000664d457821/Sources/_StringProcessing/Regex/AnyRegexOutput.swift#L227

[christopherweems]

Decided to jump in and fix this & found that it had all ready been implemented in #376! Big thanks to @Azoy.