FEATURE: move `nushell` scripts and refactor `env.nu`

[amtoine]

Related to https://github.com/goatfiles/nu_scripts/pull/8.

This PR:

• removes the hardcoded prompt functions from env.nu, thanks to https://github.com/goatfiles/nu_scripts/pull/8.
• makes env.nu a bit easier to read with newlines
• makes the "multiline prompt" prettier with colors
• uses GIT_REPOS_DIR to abstract away from GHQ_ROOT
• does not force --trash on rm to allow to delete things --permanently