Prevent Flow from checking node_modules, add Prettier config

[haydn]

Some tools try to run Flow checks on files in node_modules which have the @flow pragma. These errors aren't relevant. We can prevent this by using the declarations config:

• Don't typecheck or load files under node_modules/ unless they're imported by flow-typed files facebook/flow#869
• https://flow.org/en/docs/config/declarations/#toc-declarations

Also baked in the Prettier config we're using to avoid tools falling back to some other config.

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[haydn]

@Mxchaeltrxn Can I get your eyes on this? Just want to check the changes I've made don't mess-up for you locally.

[Mxchaeltrxn]

@haydn So I'm guessing you wanted me to try running that locally. I just tried using that configuration and running the yarn format script in package.json and had no issues!

Also, any chance this configuration would get changed in the future? I quite like having trailingComma and arrowParens to say the least (from a productivity perspective).

Anyway, I get that it might cause merge conflicts now, but just wanted to ask.

[haydn]

@Mxchaeltrxn Thanks for having a bit of a play on your machine. Yeah, I just wanted to check that still worked as expected with your setup.

I agree with both of those Prettier config options and I wish I set them from the start! Tell you what might work — we can use configuration overrides to keep the existing config for the existing files, but set new config for all new files. We can then slowly remove files from the list of exceptions as we work on them until we get to the point where all the files are using our preferred config (we should probably hold off on Editor.js for a bit because they're are lots of active PR's that touch it). I'll push another commit with that change. 👍