Improve popup menu to leave some margin with window edges (#17159)

Release Notes:

- Improved popup menu to leave some margin with window edges.

## Updates in GPUI

- gpui: Add `snap_to_window_with_margin` method to `anchored` to support
leave margin to window edges.

## Before

<img width="609" alt="before-snap-to-window 2024-08-30 222506"
src="https://github.com/user-attachments/assets/62bb6791-7c89-4558-9484-5c7b31f5e91e">

## After

<img width="698" alt="snap-to-window1 2024-08-30 222506"
src="https://github.com/user-attachments/assets/51634e79-2a95-42fe-8362-a3c7003648eb">

<img width="622" alt="snap-to-window 2024-08-30 222506"
src="https://github.com/user-attachments/assets/43a865d6-d238-4fdc-ae9d-8160b9ba7953">

crates/editor/src/element.rs

@@ -2732,7 +2732,7 @@ impl EditorElement {
                         .position(position)
                         .child(context_menu)
                         .anchor(AnchorCorner::TopLeft)
-                        .snap_to_window(),
+                        .snap_to_window_with_margin(px(8.)),
                 )
                 .with_priority(1)
                 .into_any(),