Feature Request: support type checking event listeners

[e111077]

Now that WCA supports event types, it would be great to have a flag to type check event listeners. e.g.

/**
 * @fires my-ev {CustomEvent<{foo: number}>}
 */
@customElement('my-el')
export class MyEl extends LitElement {...}
...
return html `
<!-- this should throw an incompatible type error -->
<my-el @my-ev=${(e: CustomEvent<{bar: boolean}>) => {...}}>`;
...

[web-padawan]

Related to #143