Add inset variant to ListItem (#3422)

This PR adds an inset variant to the `ListItem` component.

We're now using this inset variant for the `ListItem`s we render in
pickers.

Release Notes:

- N/A

crates/storybook2/src/stories/picker.rs

@@ -61,6 +61,7 @@ impl PickerDelegate for Delegate {
 
         Some(
             ListItem::new(ix)
+                .inset(true)
                 .selected(selected)
                 .child(Label::new(candidate)),
         )