Add test to emit custom event

I'm currently encountering a problem with svelte and I think it could be interesting to know if other framework has this caveat.

The goal of the test is to see if a custom element produced by a framework can emit a [CustomEvent](https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent).

```html

document.querySelector('my-element').addEventListener('myevent', function (e) {

});

```

The sentence would be

- The library/framework should be able to dispatch DOM events from a custom element. These DOM events could be listened from the outside.