Restrict HTML emission to specific nodes

**Feature request**: I'd love the ability to extract html from a CSS selector. Currently it seems there's no good way to do so. Perhaps like so:

```js
const extractedHtml = new HTMLRewriter().on('#my-id', {
element(e) {
e.extract();
}
}).transform(response);
```

Not sure that'd be the most appropriate syntax given the nature of extracting vs. modifying/removing.

More context: https://community.cloudflare.com/t/htmlrewriter-extract-and-serve-single-dom-node/136769