add "nuenv" hook

[amtoine]

💡 Note

greatly inspired by Direnv

this new "nuenv" hook will source any .env.nu file found in the current directory as part of a hook.

in order to be a bit more safe, i've added nuenv allow and nuenv disallow that will keep track of which "env" files are allowed to be sourced, i.e. if a file is not allowed, then you'll have to run nuenv allow first.

example usage

use nu-hooks/nuenv/hook.nu [ "nuenv allow", "nuenv disallow" ]
$env.config.hooks.env_change.PWD = (use nu-hooks/nuenv/hook.nu; hook setup)

[amtoine]

this is pretty simple and kinda the same as #888, so i'll just merge it and we can definitely improve it in the future 🙏