ui: Add submenus to ContextMenu (#45743)
This PR introduces submenu functionality for Zed's dropdown/context
menus.
<img width="600" height="830" alt="Screenshot 2025-12-27 at 2 03@2x"
src="https://github.com/user-attachments/assets/eadfcd74-07fe-4a1f-be76-11b547c16dc8"
/>
```rs
.submenu("Trigger", |menu, _, _| {
menu.entry("Item…", None, |_, _| {})
.entry("Item…", None, |_, _| {})
})
```
Release Notes:
- N/A
---------
Co-authored-by: Zed Zippy <234243425+zed-zippy[bot]@users.noreply.github.com>