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)