[scoped-custom-elements-polyfill]: HTMLFormControlsCollection overwritten with a version that does not support the iterator protocol

### Description

When loading the scoped elements polyfill, the implementation of `HTMLFormElement.elements` get replaced with a custom `HTMLFormControlsCollection`. This implementation lacks an implementation for the iterator protocol, it [throws an error](https://github.com/webcomponents/polyfills/blob/16a0a1e87f67c2604381b904b4edd8cd112e5b04/packages/scoped-custom-element-registry/src/scoped-custom-element-registry.ts#L847-L848) instead.

Furthermore, the implementation gets replaced globally. So even in a form that does not have custom elements at all, `HTMLFormElement.elements` is not iterable.

### Example

Consider the following HTML:

```html

```

### Steps to reproduce

Without the scoped registry polyfill, this prints all elements contained in this form:

```js
const formEl = document.querySelector("form");

for (const el of formEl.elements) {
console.log(el);
}
```

Loading the polyfill prior to looping over the form elements throws an error on the for/of iteration.

#### Expected behavior

No error is thrown and we can iterate over the elements in a form.

#### Actual behavior

An error is thrown.

### Version

`@webcomponents/scoped-custom-element-registry: 0.0.10`

### Browsers affected

- [x] Chrome
- [x] Firefox
- [x] Edge
- [x] Safari
- [ ] IE 11

I believe all browsers to be affected, but only tested in Chrome.