Rework ListItem and ListHeader to use slot-based APIs (#3635)

This PR reworks the `ListItem` and `ListHeader` components to use
slot-based APIs, making them less opinionated about their contents.

Splitting this out of the collab UI styling PR so we can land it to
avoid conflicts.

Co-authored-by: Nate <nate@zed.dev>

Release Notes:

- N/A

crates/ui2/src/components/context_menu.rs

@@ -255,6 +255,9 @@ impl Render for ContextMenu {
                                 };
 
                                 ListItem::new(label.clone())
+                                    .inset(true)
+                                    .selected(Some(ix) == self.selected_index)
+                                    .on_click(move |_, cx| handler(cx))
                                     .child(
                                         h_stack()
                                             .w_full()
@@ -265,8 +268,6 @@ impl Render for ContextMenu {
                                                     .map(|binding| div().ml_1().child(binding))
                                             })),
                                     )
-                                    .selected(Some(ix) == self.selected_index)
-                                    .on_click(move |_, cx| handler(cx))
                                     .into_any_element()
                             }
                         },