Document and expose configuration file

[jakobjpeters]

I would like to store the equivalent of juliaup config autoinstallchannels true in a configuration file, but face two issues:

1. It appears as if this state should be stored in .juliaup/juliaupself.json, but that file does not get updated when I change the value of autoinstallchannels even though the state is remembered by juliaup.
2. I don't want to rely on the undocumented internal storage of this configuration.

Configuration files are a very common resource for developers to declaratively setup their system, rather than relying on stateful commands. This feature would enhance the useability of juliaup.