Add an `all_members` function

Have a helper to retrieve all groups in the registry:

```gleam
/// Retrieves all subjects on the registry.
/// 
/// ## Example
/// 
/// ```gleam
/// let assert Ok(registry) = chip.start()
/// 
/// let assert [subject, ..subjects] = chip.all_members(registry)
/// ```
pub fn all_members(registry: Registry(msg, group)) -> List(Subject(msg)) {
  todo
}
```