[RRFC] exportable config definitions

[fritzy]

As explained in npm/cli#5119

Motivation ("The Why")

@Kytech writes

Since libnpmconfig and the programmatic APIs have been deprecated, there is no library or API provided by NPM that can be consumed by other packages to read NPM's configuration. libnpmconfig has been replaced with @npmcli/config, which cannot read the npm configuration on its own.

In order to allow other programs to read the npm config file using NPM's config resolution algorithm, the definitions in this repository (https://github.com/npm/cli/tree/latest/lib/utils/config) need to be made available.

There are a large number of projects that read the npm config files, but they all have to resort to writing a large amount of the resolution code themselves. Yarn and PNPM provide packages that allow their respective config files to be read by other programs, but since the deprecation of libnpmconfig and the programmatic API, NPM does not have a usable config package (as far as other applications are concerned), which has caused pain points for a number of projects, especially since the recent security issue with rc, which many packages used to roll their own solution, and are now looking to replace as a dependency.

Example

@Kytech mentions being able to retrieve the auth token.