juliaup list with json output

[JosiahParry]

Similar to how we can use juliaup api getconfig1 to get the installed julia channels as json, it would be great if we could also access the available julia channels that can be installed via juliaup list as json.

[ericphanson]

This would be useful for RegistryCI too (https://github.com/JuliaRegistries/RegistryCI.jl/pull/606)

[davidanthoff]

Agreed. I think the way to add this is something like juliaup api getchannellist1 for example.