editor: Add duplicate selection command (#21154)

Closes #4890 

Release Notes:

- Add duplicate selection command for editor

crates/editor/src/editor_tests.rs

@@ -3895,6 +3895,28 @@ fn test_duplicate_line(cx: &mut TestAppContext) {
             ]
         );
     });
+
+    let view = cx.add_window(|cx| {
+        let buffer = MultiBuffer::build_simple("abc\ndef\nghi\n", cx);
+        build_editor(buffer, cx)
+    });
+    _ = view.update(cx, |view, cx| {
+        view.change_selections(None, cx, |s| {
+            s.select_display_ranges([
+                DisplayPoint::new(DisplayRow(0), 1)..DisplayPoint::new(DisplayRow(1), 1),
+                DisplayPoint::new(DisplayRow(1), 2)..DisplayPoint::new(DisplayRow(2), 1),
+            ])
+        });
+        view.duplicate_selection(&DuplicateSelection, cx);
+        assert_eq!(view.display_text(cx), "abc\ndbc\ndef\ngf\nghi\n");
+        assert_eq!(
+            view.selections.display_ranges(cx),
+            vec![
+                DisplayPoint::new(DisplayRow(0), 1)..DisplayPoint::new(DisplayRow(1), 1),
+                DisplayPoint::new(DisplayRow(2), 2)..DisplayPoint::new(DisplayRow(3), 1),
+            ]
+        );
+    });
 }
 
 #[gpui::test]