Add prettier

[ryanto]

No description provided.

[xg-wang]

Thanks for adding Prettier! Can you also add a precommit hook to prettify code? https://prettier.io/docs/en/precommit.html

[ryanto]

We were exploring adding this to Mirage, but I got some negative feedback (see https://twitter.com/ryantotweets/status/1203065809167409154). I'm not sure if it's worth adding.

I can add the prettier check to CI so we can failures if someone opens a PR with unformatted code. How does that sound?

[xg-wang]

Pretender doesn't run lint in ci now, I think adding prettier eslint integration and make it easier to yarn eslint --fix should be enough.