Package Registry Improvements

Description

https://github.com/nushell/nupm/pull/56 added the ability to install nupm packages from a registry. Currently, we have a registry.nuon at the repository root but as we go, we'll need to make it more robust and maintainable.

Proposed steps

• Move registry.nuon to a separate repository (e.g., nushell/nupkgs)
• Allow splitting registry.nuon to separate files. E.g., each package could have its own registry.nuon and these would be gathered at the top-level registry.nuon. I think with the growing number of packages, a single file would quickly become unmaintainable.
• Add a CI job to test package installations.
• In the long run, think about stability: What if some package's CI starts failing? What if some package's URL becomes unavailable? Maybe we can allow creating snapshots: Download package files from all packages defined in the registry and distribute it as a huge zip file. Users could then download the snapshot and be 100% sure that all packages are available.

Start with the root-level registry.nuon and the package-installation work introduced by PR #56. Review how registry entries are loaded, then consider the proposed separate repository, split registry files, and CI installation checks. Done would require an agreed, maintainable registry design and a working validation path for package installations.