This is a part of #2717
Some settings didn't get getters because
- They are transient parse settings (e.g. ignore errors)
- They get propagated to args and should be checked there
`is_allow_hyphen_values_set` is a curious case. In some cases, we only
check the app and not an arg. This seems suspicious.
For the derive API, you can only call `next_display_order` when dealing
with a flatten. Until we offer app attributes on arguments, the user can workaround with
this no-op flattens.
This is a part of #1807
This clarifies the intent and prepares for other functions doing the
same, like `next_display_order`. This will then open us to name
`subcommand_help_heading` and `display_order` similar.
The deprecation is waiting on 3.1.
This is part of #1807 and #1553.
