Add text node support to FragmentInstance operations (#35630)

This PR adds text node support to FragmentInstance operations, allowing
fragment refs to properly handle fragments that contain text nodes
(either mixed with elements or text-only).

Not currently adding/removing new text nodes as we don't need to track
them for events or observers in DOM. Will follow up on this and with
Fabric support.

## Support through parent element
- `dispatchEvent`
- `compareDocumentPosition`
- `getRootNode`

## Support through Range API
- `getClientRects`: Uses Range to calculate bounding rects for text
nodes
- `scrollIntoView`: Uses Range to scroll to text node positions directly

## No support
- `focus`/`focusLast`/`blur`: Noop for text-only fragments
- `observeUsing`:  Warns for text-only fragments in DEV
- `addEventListener`/`removeEventListener`: Ignores text nodes, but
still works on Fragment level through `dispatchEvent`