[npm] configurable registry

Add a registry option to the npm publisher config so users can publish to custom npm registries instead of the hardcoded https://registry.npmjs.org/.

Locate the npm publisher configuration and the code that currently uses https://registry.npmjs.org/. Start by tracing how npm publishing options are read, then verify that a custom registry can be supplied and used instead of the hardcoded URL.

Written by the indexing model from the issue text.