Bundled Themes: Use Node.js 16.x in themes with a build process.

This results in a new `package-lock.json` file being generated that uses `lockfileVersion` 2.x.

There are no changes to built files as a result of this update.

Follow up to [56378], [56387], [56442].

Props mikestraw, whyisjake, jivygraphics.
Fixes #59187. See #56658.

git-svn-id: https://develop.svn.wordpress.org/trunk@56531 602fd350-edb4-49c9-b593-d223f7449a82