Allow plugin functions in plugin_group!

[Jengamon]

Objective

Allow plugin functions to be used inside plugin_group! declarations.

Right now, functions of signature fn(&mut App) implement Plugin, but cannot be added to plugin groups, due
to how the macro is implemented.

This fixes that.

Solution

This PR fixes the problem by introducing a block after the top-level members, labeled with fn, where the names are considered to be referencing values directly, rather than types. It looks like:

plugin_group! {
  pub struct MyFnGroup {
     // ... stuff that `impl Plugin` ...
     fn {
       :local_fn,
       outer:::outer_fn,
       #[doc(hidden)]
       internal:::hidden_fn,
    }
  }
}

Testing

I edited the doctest above the macro implementation, and made sure that passes, and the syntax is non-invasive otherwise, so overall, not much should change.

---

[github-actions]

Welcome, new contributor!

Please make sure you've read our contributing guide, as well as our policy regarding AI usage, and we look forward to reviewing your pull request shortly ✨