Implement move_line_up and move_line_down

This does not restore folds yet.

zed/src/editor/buffer/selection.rs

@@ -73,7 +73,11 @@ impl Selection {
         }
     }
 
-    pub fn buffer_rows_for_display_rows(&self, map: &DisplayMap, ctx: &AppContext) -> Range<u32> {
+    pub fn buffer_rows_for_display_rows(
+        &self,
+        map: &DisplayMap,
+        ctx: &AppContext,
+    ) -> (Range<u32>, Range<u32>) {
         let display_start = self.start.to_display_point(map, ctx).unwrap();
         let buffer_start = DisplayPoint::new(display_start.row(), 0)
             .to_buffer_point(map, Bias::Left, ctx)
@@ -93,6 +97,9 @@ impl Selection {
         .to_buffer_point(map, Bias::Left, ctx)
         .unwrap();
 
-        buffer_start.row..buffer_end.row + 1
+        (
+            buffer_start.row..buffer_end.row + 1,
+            display_start.row()..display_end.row() + 1,
+        )
     }
 }