feature: allow tracking of custom depot paths

[uncomfyhalomacro]

Scenario

Some users use a custom depot path for Julia using the JULIA_DEPOT environmental variable.

Rationale

Regardless of the reason, juliaup should track custom depot paths to allow flexible configuration and managing of julia versions on customized setups.

Use cases

• Self uninstalling of ~/.julia/juliaup and custom depots e.g. $JULIA_DEPOT/juliaup as mentioned in Should we delete .julia/juliaup at uninstall? #457
• A saved data (maybe as JSON?) of tracked custom depots for users to clean up for uncommon cases such as a broken juliaup installation.
• Customized setups where littering the HOME directory is not desirable.

[uncomfyhalomacro]

We still have this issue #661 that is not yet resolved. So this is a braindump for now until that's done.

[davidanthoff]

I don't really understand what the proposal actually is. What do you mean by "tracking"? I think this just needs more explanation of what you actually have in mind :)