allow certain npm files

[NathanPJF]

What are you trying to accomplish with this PR?

The package-lock.json and npm-shrinkwrap.json files were added to the gitignore because it was believed these should stay out of the repo.

The package-lock.json is supposed to be committed to repos. From the file's description on npmjs.org https://docs.npmjs.com/files/package-lock.json#description

It describes the exact tree that was generated, such that subsequent installs are able to generate identical trees, regardless of intermediate dependency updates. This file is intended to be committed into source repositories, and serves various purposes

As for the npm-shrinkwrap.json: we're not currently including one and have no immediate plans to. But it should not be ignored from the project.

Checklist

For contributors:

• I have updated the docs to reflect these changes, if applicable.

For maintainers:

• I have 🎩'd these changes.
• I have bumped the package.json version in a separate PR, if applicable.

[bertiful]