[Fix] functional SFC style injection support

[blake-newman]

Adds support for SFC components that have injectStyles.

Example component:

<script>
  export default {
    functional: true,
    render(h, { children }) {
      return h('div', children);
    },
  };
</script>

<style lang="scss">
  @import 'style/settings';
  @import 'style/tools';
  @import 'style/components/wrapper';
</style>

Also does not apply hook to functional components, without injectStyles. I believe that this is not needed, correct me if i'm wrong.

[yyx990803]

This piece of code will run in browser without compilation so we need to use ES5

[blake-newman]

@yyx990803 Updated to use es5

[yyx990803]

const is still es6 ;)

[blake-newman]

Damn so used to writing ES6 now :D