resize hook

I've been using this hook

```jsx
function useContentRect (ref) {
const [contentRect, setContentRect] = useState(ref.current ? ref.current.getBoundingClientRect() : {});
useEffect(() => {
if (!ref.current) return;
const observer = new window.ResizeObserver(entries => setContentRect(entries[0].contentRect));
observer.observe(ref.current);
return () => observer.disconnect(ref.current);
}, [ref]);
return contentRect;
}
```
To measure elements and react on size changes. Can be used as
```jsx
function Component () {
const ref = useRef(null);
const { width, height } = useContentRect(ref);
return

;
}
```
Looks like something like this could be a nice addition to this library. If not, feel free to just close and ignore. 🙂

The issue names no repository file, test, or entry point, so first locate how this library organizes and exports hooks. Compare the proposed behavior with existing hook patterns; done should mean the resize hook is added in the expected public location and its size-change behavior is verified.

Written by the indexing model from the issue text.