Extend `qmk setup` to handle userspace

[zvecr]

Description

Couple of choices....

1. Different subcommand?
   • qmk userspace-setup to align with the other userspace commands?
   • can always merge later?
2. Extend existing setup command
   • tutorials/docs/etc already say to run setup?
     • Default to userspace or opt in?

TODO:

• "hidden" qmk_firmware location?
  • currently platformdirs.user_data_dir('qmk_cli', 'QMK')/qmk_firmware
    • /home/me/.local/share/qmk_cli/qmk_firmware