prettier support for already-installed node.js packages

For projects which are already using gradle to manage node.js and install deps with package.json, it would be nice if our prettier config could hook into that. Maybe if it had an arg where you passed the `node` executable, and another for passing `package-lock.json` for caching, and it was on you (somehow) to make sure that the dependencies were wired correctly. Getting the ergonomics right is key, and also difficult...