Support .npmrc registry/auth settings

[alexeagle]

https://docs.npmjs.com/using-private-packages-in-a-ci-cd-workflow#create-and-check-in-a-project-specific-npmrc-file

In practice we probably just have to validate that the Bazel downloader config flag is set, and the downloader config has mirrored the settings from .npmrc.

[pedrobarco]

I think this has been fixed with https://github.com/aspect-build/rules_js/pull/475

[alexeagle]

I think we're missing a documentation update to consider it done