Configure npm registry via .npmrc #642

Affected Packages

pkg:@changesets/cli

Problem

Looks like we can only get the npm registry from packageJson.publishConfig or process.env.npm_config_registry when running npm info. Is there a way to get npm registry from .npmrc file?

Also, is npm_config_registry safe to rely on? Because it's not documented.

Proposed solution

Read npm registry in the following priorities:

1. packages/my-package/packageJson.publishConfig
2. packages/my-package/.npmrc
3. root/.npmrc