Allow specifying subcommands via the poise::command macro

[elkowar]

Currently, specifying subcommands requires you to manually insert them into the Command struct, commonly done via

Command {
  subcommands: vec![a(), b(), c()],
  ..foo()
}

which is quite cumbersome, especially if you're splitting up your commands into separate modules.

To improve this, I'd propose adding a subcommands attribute to the poise::command macro, which would allow specifying subcommands on the parent command like this:

#[poise::command(
  slash_command,
  subcommands(a, b, c)
]
pub fn foo(_: Context<'_>) -> Result<(), Error> {
  Ok(())
}

[kangalio]

b740663