New rule: Prefer `async` functions over `generator` functions

[deanmarano]

Hi there!

I was thinking a new rule for ensuring consistent usage of the async syntax:

Bad

default class extends Component {
  @task *fetchUser() {
    let user = yield this.currentUser.fetch();
    // ...
  }
}

Good

default class extends Component {
  fetchUser = task(async() => {
    let user = await this.currentUser.fetch();
    // ...
  }
}

Would something like this fit in your plugin? I'd be happy to take a stab at implementing.