Support multiple registries

[Barbapapazes]

Describe the feature

Hello,

Currently, there is a default registry for unjs template. To use a custom registry, we have to use --registry and pass a long URL like https://raw.githubusercontent.com/unjs/giget/main/templates.

I'm currently creating my own registry. To fetch from my registry, I've to pass the long URL which is annoying. I could put it in a var env but this will overwrite the unjs registry.

Using a config file with unjs/rc9, we could easily support multiple registries. We could also support multiple registries in var env using a separator like a comma.

If it's ok for you, I'm ok to try to implement this feature.

Additional information

• [X] Would you be willing to help implement this feature?