Enhancement: show error for state objects in functional components

[richardtallent]

• I have searched through existing issues
• I have read through docs
• I have read FAQ

Info

• Platform: OS X
• Vetur version: 0.12.3
• VS Code version: 1.23.1

Problem

When a component has the functional attribute, it would be nice if Vetur recognized this and showed an error if the developer has data or computed objects in the exported script, as functional components have no state.

I don't know enough about functional components to know if there are other restrictions around lifecycle events, watchers, etc.

Reproducible Case

<template functional>
  <h1>{{ foo }}</h1>
</template>
<script>
export default {
	data: () => ({
		foo: 1,
	}),
}
</script>