Inset ContextMenu headers (#9197)

This PR insets the headers within `ContextMenu`s to give them some more
breathing room.

#### Before

<img width="347" alt="Screenshot 2024-03-11 at 4 13 31 PM"
src="https://github.com/zed-industries/zed/assets/1486634/73a56d68-d40e-4396-b584-f443197b69d6">

#### After

<img width="354" alt="Screenshot 2024-03-11 at 4 12 43 PM"
src="https://github.com/zed-industries/zed/assets/1486634/44c12a07-0784-4c94-b194-245f5cf94b2b">

Release Notes:

- Added padding to headers in context menus.

crates/ui/src/components/list/list_sub_header.rs

@@ -21,6 +21,11 @@ impl ListSubHeader {
         self.start_slot = left_icon;
         self
     }
+
+    pub fn inset(mut self, inset: bool) -> Self {
+        self.inset = inset;
+        self
+    }
 }
 
 impl RenderOnce for ListSubHeader {