In prep for handling the above-viewport case in
https://github.com/zed-industries/zed/pull/57632, which currently only
handles below case.

This PR adds `ListState::item_is_above_viewport` and
`ListState::item_is_below_viewport` methods, which report whether a
given list item is entirely outside the current viewport. Both return
`None` when the list has not measured enough layout to answer.

Release Notes:

- N/A