Prevent crash if `.julia/registries` does not exist

In the current implementation, upon doing `juliapkg.add()` in an environment where `.julia/registries` does not yet exist, `juliapkg` crashes.
This may seem like an edge case, but is a common occurrence for CI and testing where Julia environments are commonly freshly instantiated and have not yet been used before using `juliapkg`, e.g. with `julia-actions/setup-julia@v3`.
My current workaround is to create the folder from the test script if it doesn't exist, but it would be better if `juliapkg` handles this without crashing. Hope that can be arranged!

Reproduce the crash by running juliapkg.add() in a fresh Julia environment where .julia/registries is absent, then trace the registry handling from that entry point. Done means juliapkg.add() completes without requiring the caller to create the directory, including in a fresh CI-style environment.

Written by the indexing model from the issue text.