Support arrow functions in no-setup-in-describe rule

[crcatala]

While Mocha discourages the arrow function as arguments to describe, etc. would it still be possible to get the no-setup-in-describe rule to work with arrow functions?

For example:

describe('foo', function () {
  a(); // `no-setup-in-describe` rule works as intended
});

describe('foo', () => {
  a(); // `no-setup-in-describe` rule does not seem to apply here
});