[modular] Decouple help generation from clap

[epage]

Please complete the following tasks

• I have searched the discussions
• I have searched the existing issues

Clap Version

master

Describe your use case

We are looking to modular clap

• We can look for opportunities to shrink clap
• We can share logic with other argument parsers

One aspect of this will be to have a general reflection API. A good stepping stone test case is for help generation to not depend on clap but a clap_reflect. This will allow us to test the reflection API out before making it public.

Describe the solution you'd like

1. Create an internal reflection API in clap
2. Port the help generation to it
3. Work to move these out into a clap_reflect and clap_help crates

Alternatives, if applicable

No response

Additional Context

No response