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.