Add observer methods to fragment instances (#32619)

This implements `observeUsing(observer)` and `unobserverUsing(observer)`
on fragment instances. IntersectionObservers and ResizeObservers can be
passed to observe each host child of the fragment. This is the
equivalent to calling `observer.observe(child)` or
`observer.unobserve(child)` for each child target.

Just like the addEventListener, the observer is held on the fragment
instance and applied to any newly mounted child. So you can do things
like wrap a paginated list in a fragment and have each child
automatically observed as they commit in.

Unlike, the event listeners though, we don't `unobserve` when a child is
removed. If a removed child is currently intersecting, the observer
callback will be called when it is removed with an empty rect. This lets
you track all the currently intersecting elements by setting state from
the observer callback and either adding or removing them from your list
depending on the intersecting state. If you want to track the removal of
items offscreen, you'd have to maintain that state separately and append
intersecting data to it in the observer callback. This is what the
fixture example does.

There could be more convenient ways of managing the state of multiple
child intersections, but basic examples are able to be modeled with the
simple implementation. Let's see how the usage goes as we integrate this
with more advanced loggers and other features.

For now you can only attach one observer to an instance. This could
change based on usage but the fragments are composable and could be
stacked as one way to apply multiple observers to the same elements.

In practice, one pattern we expect to enable is more composable logging
such as

```javascript
function Feed({ items }) {
  return (
    <ImpressionLogger>
      {items.map((item) => (
        <FeedItem />
      ))}
    </ImpressionLogger>
  );
}
```

where `ImpressionLogger` would set up the IntersectionObserver using a
fragment ref with the required business logic and various components
could layer it wherever the logging is needed. Currently most callsites
use a hook form, which can require wiring up refs through the tree and
merging refs for multiple loggers.