Lint against using `class` and `.extend` together (except with mixins)

[pzuraq]

In general, the following pattern should be avoided:

export default class PersonComponent extends Component.extend({
  fullName: computed({
    get() {
      return `${this.firstName} ${this.lastName}`; 
    }
  })
}) {
  firstName = 'Diana';
  lastName = 'Prince';
}

However, it is valid to pass a number of mixins to .extend()

export default class PersonComponent extends Component.extend(FullNameMixin, OtherMixin) {
  firstName = 'Diana';
  lastName = 'Prince';
}