Add option to enable --use-on-cd in automatic installation script

[mrmckeb]

Hey @Schniz!

We're recommending fnm as a part of our onboarding docs, and it would be great if there was an option (or flag) to automatically set up fnm with --use-on-cd, as a one-liner.

If you're open to it, I'd be happy to pick this up too.

[Schniz]

I’m considering supporting default arguments in ~/.config/fnm.toml like

[env]
use_on_cd = true

what do you think about this? Then it can be a part of a shared dotfiles or a script!

[Schniz]

We can also make the installation process more configurable or even use use-on-cd by default

[mrmckeb]

I've given this some more thought. I personally feel that defaulting to --use-on-cd might make sense.

Would you be happy with that approach? I think for many users, that would be preferable - but I could be wrong 😆