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/file_finder2/src/file_finder.rs

@@ -719,7 +719,7 @@ impl PickerDelegate for FileFinderDelegate {
             self.labels_for_match(path_match, cx, ix);
 
         Some(
-            ListItem::new(ix).selected(selected).child(
+            ListItem::new(ix).inset(true).selected(selected).child(
                 v_stack()
                     .child(HighlightedLabel::new(file_name, file_name_positions))
                     .child(HighlightedLabel::new(full_path, full_path_positions)),