While working on https://github.com/zed-industries/zed/pull/63343, I ran
into a problem with the pending key binding indicator. Hovering pauses
its countdown, but pressing the next key made `on_hover` report false
even though the mouse had not moved. The countdown then resumed while
the pointer was still over the indicator.

`on_hover` currently follows GPUI's normal hover rules. GPUI clears
hover after keyboard input so the item under a stationary mouse does not
interfere with keyboard navigation, for example list popover navigation
with keyboard.

This PR adds `HoverListenerMode` and `hover_listener_mode`. Callers that
need to track the pointer across key presses can opt in like this:

```rs
div()
    .hover_listener_mode(HoverListenerMode::InputModalityIndependent)
    .on_hover(|is_hovered, window, cx| {
          // ...
      })
```

`InputModalityAware` remains the default.

Release Notes:

- N/A