[RRFC] Support multiple registries per scope in .npmrc

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 #427

Start by reading the .npmrc configuration model described in this issue and the related proposal in #427. Define how multiple project- and group-level registries should coexist under one scope, then document the expected configuration and resolution behavior as the completed RFC.