Update dependencies, revise lint rules

[edsrzf]

This updates dependencies, including some major versions:

• Prettier v2 - some changes to formatting and now arrowParens: "avoid" is required if we want to keep our single-parameter arrow functions from being reformatted.
• ESLint v7 - no real breakage for us here.

And while we're breaking things, it makes some changes to rules. Most notably:

• import/no-default-export is enabled to prevent use of default exports.
• All accessibility rules from create-react-app are enabled.

Finally, I've also moved the rules from index.js to .eslintrc.js, which makes it easier to use this repo's rules to lint itself and validate the configuration.

[unkillbob]

Just had a scan of the rules changes, LGTM 👌 One question - do the new rules, e.g. the a11y ones, require fixing things in monocle etc? Have you had a check to see how big a task that is to upgrade?

[edsrzf]

I've got a Monocle change ready for everything but the a11y rules. I see 15 errors due to those rules, which doesn't seem unreasonable.

I'll probably override most of them with an eslint-disable comment for now since the proper fixes may be a bit more involved, but I'll evaluate that on a case-by-case basis.

[ivyfu77]

Can I ask an arrow function defined like const someName = () => {...} will be detected as variable or function?

[edsrzf]

According to the docs, it's any named function declaration or named function expression. So no, I don't believe it covers arrow functions like that.

I believe arrow functions are why I ended up allowing PascalCase for variables, since React component names often start with capital letters. eg:

const Component = () => ...