Implement move_line_up and move_line_down

This does not restore folds yet.

zed/src/editor/display_map/fold_map.rs

@@ -169,6 +169,13 @@ impl FoldMap {
         false
     }
 
+    pub fn to_buffer_offset(&self, point: DisplayPoint, app: &AppContext) -> Result<usize> {
+        let mut cursor = self.transforms.cursor::<DisplayPoint, TransformSummary>();
+        cursor.seek(&point, SeekBias::Right);
+        let overshoot = point.0 - cursor.start().display.lines;
+        (cursor.start().buffer.lines + overshoot).to_offset(self.buffer.read(app))
+    }
+
     pub fn to_display_offset(
         &self,
         point: DisplayPoint,

zed/src/editor/display_map/mod.rs

@@ -179,6 +179,11 @@ impl DisplayPoint {
             .to_buffer_point(self.collapse_tabs(map, bias, app)?.0))
     }
 
+    pub fn to_buffer_offset(self, map: &DisplayMap, bias: Bias, app: &AppContext) -> Result<usize> {
+        map.fold_map
+            .to_buffer_offset(self.collapse_tabs(map, bias, app)?.0, app)
+    }
+
     fn expand_tabs(mut self, map: &DisplayMap, app: &AppContext) -> Result<Self> {
         let chars = map
             .fold_map