Feature request: ability to add custom `InstalledChannels` without editing the `~/.julia/juliaup/juliaup.json` file

I'd like to be able to add custom `InstalledChannels`. However, I don't want to edit the `~/.julia/juliaup/juliaup.json` file. Instead, my preferred interface would be to create a new file called `~/.julia/juliaup/channels.d/dilum_custom_channels.json`. And then `juliaup` will automatically detect any `*.json` files in the `~/.julia/juliaup/channels.d/` directory and add those channels.

If multiple channels are detected with the same name (e.g. `~/.julia/juliaup/juliaup.json` and `~/.julia/juliaup/channels.d/dilum_custom_channels.json` both have a channel named `release`), `juliaup` should throw an error.

Start by tracing how juliaup reads ~/.julia/juliaup/juliaup.json and constructs InstalledChannels. Define the loading behavior for *.json files under ~/.julia/juliaup/channels.d/, including duplicate channel names, then verify that custom channels are detected and duplicates produce an error.

Written by the indexing model from the issue text.