Add detection of subRender methods to prefer-stateless-function

[jseminck]

Related to: #1235 - though the issue wasn't yet labelled with new rule. If we want it, it's available.

[ljharb]

What about closed-over functions (in all component types) that return jsx?

[jseminck]

Could you elaborate with an example, please?

I could only think of this additional case, which indeed we should support as well:

render() {
  const mySubRenderFn = () => <div>{this.state.foo}</div>
  
  return (
    <div>{mySubRenderFn()}</div>
  )
}

[ljharb]

function shouldBeComponent() { // this should warn, because it should be a component
  return <div />;
}

function SFC(props) {
  return <div>{shouldBeComponent(props.foo)}</div>;
}
SFC.propTypes = {
  foo: PropTypes.any
};

[ljharb]

(Related to #1229)

Let's rebase this and make sure it's still valid?