class MyClass extends HTMLElement is required

Usage of:
```js
class TextField extends HTMLElement
```
is currently required.

I think this shouldn't be required. 
Using `@element text-field` or 
```ts
declare global {
    interface HTMLElementTagNameMap {
        "text-field": TextField;
    }
}
```
should be sufficient.

This would allow support for libraries like vue and svelte.

Is there a reason for this behavior?