project_panel: Only show sticky item shadow when list is scrolled (#34050)

Follow up: https://github.com/zed-industries/zed/pull/34042

- Removes `top_slot_items` from `uniform_list` in favor of using
existing `decorations`
- Add condition to only show shadow for sticky item when list is
scrolled and scrollable

Release Notes:

- N/A

crates/ui/src/components/indent_guides.rs

@@ -147,6 +147,7 @@ mod uniform_list {
             &self,
             visible_range: Range<usize>,
             bounds: Bounds<Pixels>,
+            _scroll_offset: Point<Pixels>,
             item_height: Pixels,
             item_count: usize,
             window: &mut Window,