Prettier should be pinned according to its docs

[mfisher87]

I think we need a package.json for this project to mitigate this issue described in the prettier install docs:

https://prettier.io/docs/en/install.html

Note: If you forget to install Prettier first, npx will temporarily download the latest version. That’s not a good idea when using Prettier, because we change how code is formatted in each release! It’s important to have a locked down version of Prettier in your package.json. And it’s faster, too.