Add package engines

[jclem]

This adds the following to package.json and docs/package.json:

"engines": {
  "node": ">=12",
  "npm": ">=7"
}

Node 12 works with this project and since it's the oldest maintenance LTS version of Node.js, it seems like a reasonable choice.

The docs/package-lock.json has also been updated to v2, now that we're using npm>=7, which uses the newer lock file format.

[changeset-bot]

🦋 Changeset detected

Latest commit: 6ef39fb

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/react	Patch

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[github-actions]

size-limit report 📦

Path	Size
dist/browser.esm.js	61.64 KB (0%)
dist/browser.umd.js	62 KB (0%)

[colebemis]

Thanks, @jclem!