FEATURE: add automatic scripts and default config checks and pulls to `nushell`

[amtoine]

Related to #45.

This PR:

• removes the script in lib as they all moved to https://github.com/goatfiles/nu_scripts
• add checks to
  • automatically pull the default config file it it does not exist from the nushell repo
  • automatically pull the GOAT scripts if the repo has not been already cloned

This will hopefully make the nushell config much more robust if config files are missing :ok_hand: