Fix first/last item margin on scroll (#8880)

![output](https://github.com/zed-industries/zed/assets/5518/e39a3600-99c4-4d3c-baee-efd53a474f38)

Before:

https://github.com/zed-industries/zed/assets/5518/f7a4563a-504a-4a41-bfd4-21e9439cd02b

After:

https://github.com/zed-industries/zed/assets/5518/0ba41527-46fd-404f-8207-1b8c5cf37434


Release Notes:

- Fixed first and last item margin when scroll view has padding

crates/gpui/src/elements/list.rs

@@ -241,7 +241,7 @@ impl ListState {
             let mut cursor = state.items.cursor::<ListItemSummary>();
             cursor.seek(&Count(ix + 1), Bias::Right, &());
             let bottom = cursor.start().height + padding.top;
-            let goal_top = px(0.).max(bottom - height);
+            let goal_top = px(0.).max(bottom - height + padding.bottom);
 
             cursor.seek(&Height(goal_top), Bias::Left, &());
             let start_ix = cursor.start().count;