experimental: clean multishells on shell exit

[Schniz]

This PR adds --delete-on-exit (name TBD) to allow to remove the symlinks from the multishells directory.
This was not a concern for me but I see many users that complain about it. It does introduce some complexity that I am not sure about tbh.

Fixes #696

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[Schniz]

the main issue with this solution is that it will only work if the shell was closed cleanly. I believe this won't be the case most of the time.

Maybe another way we can do it is by outputting the PID as a part of the multishell, in the fnm env, making it more dynamic. Unfortunately that will make --json fail

another option is to make the setup be something like fnm env --pid=$$ if you want to be able to delete it.

bah

no good option