Is Arg::validator_regex even needed anymore?

[epage]

It was proposed for addition in #1968. That included a workaround but the motivating reason for baking it in was yaml support but that was deprecated in #3087.

With #3732, we are getting support for users to define reusable parsers / validators, rather than having every kind baked in. Users can provide their own validators or a clap_regex crate could be created to provide a general one.

The runtime costs of regex support are only paid for by those enabling the feature. However, there are usability challenges with clap due to how large the API is. If we could cut the Arg::validator_regex, regex feature, and RegexSet, that will help focus the clap API. On top of that, we are deprecating the other validator* functions and this would stand out.

[epage]

I had considered adding a RegexValueParser but I wasn't quite happy with the generic error message and the API and realized it could just as well be supported outside of clap as inside and it would reduce the API burden of clap. This led me down this path.