Event listener props should allow an object with `handleEvent` method

[Alec Larson]

See this page from the docs.

If you're not using decorators, you can customize event listener options by passing an object to the event listener expression. The object must have a handleEvent() method and can include any the options that would normally appear in the options argument to addEventListener().

render() {
  return html`<button @click=${{handleEvent: () => this.onClick(), once: true}}>click</button>`
}