New rule: arrow-body-style

[textbook]

Arrow functions that just return a value have two options:

1. () => value; or
2. () => { return value; }.

If they do anything other than just return, the braces are not optional. Therefore, in line with principle 2, perhaps we should always encourage the second option.

[textbook]

arrow-parens felt like an easier sell as they're required in all but one case:

const noParams = () => "foo";
let oneSimpleParam = param =>  "foo";
oneSimpleParam = (param) => "foo";
const oneDestructuredParam = ({ foo, bar }) => "foo";
const oneRestParam = (...params) => "foo";
const multipleParams = (first, second) => "foo";

but arrow-body-style usage is more mixed. However, the common confusion around returning objects from arrow functions makes me lean towards inclusion:

const returnsObject = () => ({ foo: 123 });
const returnsUndefined = () => { foo: 123 };
const syntaxError = () => { foo: 123, bax: 456 };