sorbet: Commands can define subcommands

- The Args Tapioca compiler only detected methods via
  `args.methods(false)`, which didn't include `subcommand` for commands
  that define subcommands as they're dynamically generated like in
  https://github.com/Homebrew/brew/blob/e2c800cddc7945890baf0925915407661eb960aa/Library/Homebrew/cmd/services.rb#L24.
- This caused errors when typechecking tests like `services_spec.rb`
  that call `args.subcommand` on the `services` command.