feat(dart_cli): offer a mason brick that can generate new commands

[Luckey-Elijah]

Description
There is a bit a boilerplate when writing new args Commands. When using the generate dart_cli template I would like to be able to quickly and easily add new commands to the command runner (and other commands as sub commands).

I would like a mason brick to be available to

Perhaps this could be a feature bundled with very_good_cli directly, have the dart_cli template ship with a brick, or publish a brick that is compatible with the dart_cli template to brickhub.dev.