[Registry] Ability to register aliases for custom registries

[lukasIO]

Right now the docs state that components can be installed directly from a custom registry's url like so :

shadcn add http://example.com/r/hello-world.json

I'd love to see an option that minimises the burden on the user for custom registries to not have to type out the url each time they want to add a new component.

Ideally users of a custom registry would simply do something like

shadcn add @example/hello-world

where @example maps to the custom registry.

I can think of two ways of achieving this:

• provide something along the lines of a shadcn add-registry http://example.com/registry.json command that will automatically map @example/* to http://example.com/r/*.json internally
• have a global store (npm like) where registry creators can register their registries and once registered users get the @example/* mapping automatically from the global store

would of course be nice if this was combined with the folder structure proposed in #6673