Update prettier to v2.x

[nicojs]

Update prettier 1.19 -> 2.x

This is not only helpful for this repo itself, but also for everyone using json-schema-to-typescript, since prettier is a dependency of this repo.

One of the changes in prettier 2, is that arrowParens now default to always rather than avoid. I've kept this setting at avoid, because it would have mean 89 fixes needed to be done. Please let me know if you want me to set it to avoid.

[bcherny]

Thanks for the contribution!