Add automatic formatting via prettier

[marvinhagemeister]

Long time coming 🎉

• Add prettier
• Format all source code
• Apply formatting via git hooks automatically
• Make sure that it has no effect on size 🎉
• Update eslint config

[coveralls]

Coverage increased (+0.002%) to 99.889% when pulling a1118c8 on prettier-2 into 5eee583 on master.

[prateekbh]

Reviewed package.json, git hooks LGTM.
Added a nit

[andrewiggins]

Whew! 😅 Went through the files. Things look good! So excited to be able to start using my editor's formatter again (my default VSCode formatter is Prettier).

I have some changes I'll submit in a follow up PR to clean up some gnarly lines I saw. Figured it made since to do in a follow up cuz they somehow save bytes and this PR should be about not changing the size at all, to prove Prettier doesn't do that.

[andrewiggins]

Oh! I also confirmed on my machine that this PR doesn't change gzip size for any of our modules