Force `es5` trailing commas

[maraisr]

We should configure prettier to es5 trailing commas. Why? makes diffs and reordering so much easier.

[dgreif]

We've implemented this in github/github and are happy with the results. @maraisr do you think this would be worthy of a breaking change for this package if we made the change here?

[maraisr]

Hi @dgreif thank you for that. I also noticed we turned this setting to all within the prettier config. And yeah been super happy with the results. I think this is worthy of a breaking change yes.

[dgreif]

Thanks for pointing out the prettier change! Would you mind opening a PR for this? I'm happy to approve, as long as no one else on my team objects, and we can cut a breaking release