[RRFC] Support multiple registries per scope in .npmrc

[temez]

Motivation ("The Why")

The current .npmrc configuration model allows specifying only one registry per scope, which is too limiting for more complex setups.

Example

We use a self-hosted GitLab instance as our internal NPM registry provider. GitLab supports publishing packages at different levels:

• Project-level registry — each project has its own NPM registry.
• Group-level registry — a registry that aggregates all packages from child projects in the group.
• Instance-level (global) registry — exposes packages using the group name as the scope, and the project name as the package name.

Suppose we have the following structure:

• project-a published to: https://gitlab.example.com/api/v4/projects/1/packages/npm/
• libraries/* packages published to: https://gitlab.example.com/api/v4/groups/6/-/packages/npm/

This setup introduces limitations for consumers:

• We want to use a single shared scope, such as @our-org/*, across all packages in our organization.
• Some packages are published to project-level registries, while others use group-level registries.

References

• Related/Reference to [RRFC] npmrc file improvements. #427