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/picker2/src/picker2.rs

@@ -271,7 +271,6 @@ impl<D: PickerDelegate> Render for Picker<D> {
                                 },
                             )
                             .track_scroll(self.scroll_handle.clone())
-                            .p_1()
                         )
                         .max_h_72()
                         .overflow_hidden(),