Lint `package.json`s

[sndrs]

our old plugin enforced fields in published package.jsons.

now that we only have the one's on disk in the package dirs, make sure they have everything we want

e.g.

• https://npmpackagejsonlint.org
• https://github.com/ext/npm-pkg-lint
• https://www.npmjs.com/package/syncpack