Add objective error quality comparison

[sibelius]

Flow 66 improved a lot error messages

https://medium.com/flow-type/better-flow-error-messages-for-the-javascript-ecosystem-73b6da948ae2

I think it got better than Typescript

[niieani]

It's gotten much better (I use Flow every day), but I still think the errors are much harder to understand than those TypeScript (that one I use during the weekends 🤡).

The main reason, IMO, is the relatively higher complexity of the Flow type system (inference), plus the position of errors is still quite often odd/off. Errors in complex types can be super hard to understand and debug.

Ideally, we'd have some comparison of the same errors in both languages. I'm open to adding another set of files to this repo that shows exactly that. Perhaps we could simply generate those files (maybe with jest snapshots?) so its easy to update them with new versions of both systems.