feat: add new `npm merge` command

[darcyclarke]

Summary

Many users have asked for us to support, in npm itself, a mechanism to facilitate the same conflict resolution for package-lock.json diffs similar to the features found in https://github.com/npm/npm-merge-driver

Exit Criteria

• [ ] implement new npm merge (or another name) command to facilitate the same functionality as npm-merge-driver
• [ ] Tests are written to reflect new capabilities
• [ ] Documentation is updated to reflect new capabilities

[ljharb]

It seems like it'd be ideal for this feature to also be able to handle all previous lockfile iterations - npm 6 included?