[EXPERIMENTAL] Run prettier --write

[rreusser]

This PR is an alternative to #1371 using prettier, which seems to be an alternative to eslint that people are pretty excited about. As opposed to eslint, prettier does not 'fix' any code or throw errors. Instead, it just 100% rewrites it from the AST so that you have no choice over formatting. Which is kind of the point of enforcing formatting in the first place.

Just submitting for the sake of discussion. Presumably this would be a precommit/prepublish hook instead of a yes/no pass/fail step.

[rreusser]

prettierlists itself as beta and notes that it works fine but may undergo a few tweaks before it's fully stable. I saw someone from facebook saying they'd be fine using it in production if just a couple particular PRs were reviewed/merged.

[rreusser]

Closing this. prettier-standard-formatter looks interesting, but seems maybe best to wait just a moment for prettier to stabilize.