For those that want the original behavior, you can usxe
`arg.help_heading(Some("FLAGS"))` on your flags. Limitations:
- This will not give you a special sort order
- This will not get a `[FLAGS]` added to usage
For templates, we removed `{unified}` and `{flags}`. To help people
catch these, a debug_assert was added.
I'm unsure but I think there might be a change in behavior in calcuating
when to show `[OPTION]` in usage. The old code only looked at
`required` while flags looked only at arg groups. We now look at both.
Ideally we'd add these in `_build` and remove special casing for
no-groups except in the sort order of groups. I feel like thats best
left for later.
This also reduced the scope of `App`s public API.
`get_*_with_no_heading` seemed a bit specialized to be in the public
API. #2853 looks at splitting it out into its own PR.
BREAKING CHANGE: Multiple
- `UnifiedHelpMessage` removed
- `{flags}` and `{unified}` are removed and will assert when present.
- `get_*_with_no_heading` removed
Fixes#2807
