Stablize `ArgMatches::grouped_value_of` Tracking Issue

[epage]

Original request: #1026

Original PR: #2297

Feature flag: unstable-grouped

Known issues

• Open question: how should this be exposed in clap_derive (see also clap_derive: Vec/Option<Vec> behavior is inconsistent with other types #1772)
• Open question: update for new get/remove API (ArgMatches.grouped_values_of() does not support groups #4372)
• Evaluate new names as this can be confusing with ArgGroup, occurrence is normally what we call this
• Function is not documented (Add documentation for grouped_values_of #3551)

[epage]

For _t and _os variants, should we wait on #2683 which will remove the need for them? Or at least not block this feature on them?

[pksunkara]

I think that's a good idea, since I am getting the impression that we will probably be working on #2683 as the next big thing after v3 release. Unless you have different plans?

[epage]

Unless you have different plans?

That and a couple other changes that will lay the ground work for help, parser, and other validation changes. I don't think we need to cram them all in but should keep an eye for changes that make it easier to get them in later. #2911 is one example because it means we can attach the reflection traits to the AppParser without making them &mut to allow building since the type system will enforce that building has occurred.