What's the problem this PR addresses?

The $npm_package_name and $npm_package_version variables weren't exposed in order to prevent the explosion of variables we would need to put in the environment, and avoid ambiguities when a package wouldn't list them and would accidentally "inherit" those from other scripts.

After more consideration, I think it's reasonable to add those two since every package will have them, so there's no ambiguity issue.

Fixes #1659

How did you fix it?

Added the relevant environment variables, plus documentation, plus test.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.