--only-setup cmd arg

[KANAjetzt]

Needs to be merged after #89

Adds cmd arg:

• --only-setup
  Allows to setup the mod loader without user notification, or auto restart ( upcoming PR ).

Request from Mythic ( Thunderstore ):

Mythic — 30/01/2023 09:39
how about would it be possible to support running just the installation step without restarting, but rather shutting down once it's complete?
that way we can use that as the "installer" on the manager, and you don't need to worry about CLI arg handling in the restart phase

Mythic — 30/01/2023 09:43
and if that can return an appropriate exit code, (0 for success, non-zero for fail), that'd be great too
then we can have the manager just run that for install

[KANAjetzt]

I think I messed that up