Add support for npm based package-lock.json

[leondmello]

Summary

Add support for npm based package-lock.json

Use Cases

With the latest version of shakapacker (v8), Rails applications can now use npm as a package manager. This PR makes it so that those apps can leverage the rails-assets buildpack and fixes #393

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: leondmello / name: Leon Dmello (e7c1abd, af9bd5f)

[sophiewigmore]

@leondmello hey! thanks for the PR. I think that the overall idea totally make sense, I'm just thinking through how this will all fit together in the Ruby buildpack (we will need to add the NPM Install buildpack to an order group). In the mean time, would it be possible to add a basic app and matching integration test, that represents this use case?

We have a README in the 6.1 app which explains how we generated the app, but also feel free to copy that app, and make modifications as needed. The matching integration test is here. I'd expect something similar for this use case in the tests, but using NPM Install rather than Yarn Install
Let me know if you have questions

[leondmello]

Sorry, I missed your comment, will work on that soon

[leondmello]

@sophiewigmore Can you add a semver label, not sure if the integration tests will run or not if that action is failing