Add npm shrinkwrap file.

[theiawolfe]

Since most people and many CIs do the initial install of yarn by
running npm install yarn, please include the npm shrinkwrap file.

Summary
Adds the file 'npm-shrinkwrap.json'

This will prevent issues such as #3850 when installing yarn via npm, as suggested in #3853

[jiripospisil]

Be aware of this however npm/npm#17185.

[arcanis]

Hey, thanks for this PR! However, I'm not sure this is the right way to solve this issue. We should ideally publish the single-file releases, which wouldn't be affected by any dependency issue.

[bestander]

I agree with @arcanis.
Yarn is not ready to maintain the shrinkwrap file in its repo because all developers are using Yarn for development.
As @arcanis described to avoid subdependencies breaking Yarn it is better to introduce a prepublish script that would bundle yarn.js and publish only this file.

I'll close this PR but looking forward for a PR with the prepublish script 🥇