[suggestion] juliaup add <channel> <version>

Say I want to peg a channel named `dev` to the latest `1.7~aarch64` version. With the existing `add` and `link`, I don't see that as possible. Consider adding `juliaup add `:

- `juliaup add dev 1.7~aarch64`
-- downloads version `1.7~aarch64` if needed
-- adds an entry for `dev` pointing to `1.7~aarch64`

- `juliaup add prod lts`
-- downloads version `lts` if needed
-- adds an entry for `prod` pointing to `lts`

Start by tracing the existing `juliaup add` and `link` command entry points. Check how channel names and versions are currently downloaded and registered, then define the behavior for `juliaup add ` so both examples work and existing commands remain compatible.

Written by the indexing model from the issue text.