Make CustomElements extends work

[F0rsaken]

After some research, it turns out to extend native HTML elements, you need to make a class that extends them, example:

export class CoolButton extends HTMLButton {
    ...
}

window.customElements.define('cool-button', CoolButton, { extends: 'button' });

You need to do both: extend a class and add extends in options.

I think we should postpone fixing this to next version

EDIT 17.12.2021:
I think this feature is not required now and I see no use case for it. We should stall this issue for now.