Add prettier for code formatting

[gr2m]

Now that the number of contributors is growing, I'd like to introduce prettier for code formatting

Here is the syntax I usually use. I don't want us to customize prettier rules. That way people can get mad at prettier and not us if they don't like any of the formatting 🤣

https://github.com/gr2m/github-app-slack-demo/blob/47fc915ff72c7b0a3e4f9b26761c18c303cd5152/package.json#L15

I don't want to add it to our CI and bother our contributors. Instead, we can silently fix formatting directly in the main branch using a dedicated app later. I'll take care of that once this issue is resolved. But in a nutshell

1. Register a single purpose GitHub app with contents:write permission, webhooks disabled.
2. Install the app on this repository
3. Add the App ID as variable and private key as secret to this repository
4. Create a new workflow that is triggered by push to main
   1. Use https://github.com/actions/create-github-app-token to create an installation access token
   2. Use that token to check out the repository
   3. run the lint:fix npm script
   4. Commit the changes (if any) with style: prettier and push them
5. Using repository rules, add the registered github app to the allowed actors that can bypass branch protections
6. Profit 🎉

Here is a format.yml workflow for reference:
https://github.com/gr2m/github-app-slack-demo/blob/47fc915ff72c7b0a3e4f9b26761c18c303cd5152/.github/workflows/format.yml