ui: Add a TreeViewItem component (#39253)

A new (and very simple, for now) `TreeViewItem` component in the set.

<img width="500" height="1712" alt="Screenshot 2025-10-01 at 8  59@2x"
src="https://github.com/user-attachments/assets/c2de1585-7b42-4d20-a749-30d93898ae37"
/>

Release Notes:

- N/A

crates/ui/src/components.rs

@@ -38,6 +38,7 @@ mod tab;
 mod tab_bar;
 mod toggle;
 mod tooltip;
+mod tree_view_item;
 
 #[cfg(feature = "stories")]
 mod stories;
@@ -82,6 +83,7 @@ pub use tab::*;
 pub use tab_bar::*;
 pub use toggle::*;
 pub use tooltip::*;
+pub use tree_view_item::*;
 
 #[cfg(feature = "stories")]
 pub use stories::*;