Accept a trait in the return of a `#[command]`

[chippers]

Is your feature request related to a problem? Please describe.
Accept a trait in the return of a #[command] function. Default implementations will include () and Result.

Describe the solution you'd like
We should implement this at the same time as #1665 and #1622 to change all aspects of #[command] codegen in a single release. It's likely I will implement this functionality this weekend.

Describe alternatives you've considered
/* ... */

Additional context
All 3 PRs will have the end result of a small change of with_window not doing anything as all things needed for knowing how to handle window will be moved to a trait. It's not a breaking change, but we will not be reading it and it should be removed from all examples and documentation