Implement add_cursor_above and add_cursor_below for buffer

zed/src/editor/buffer/mod.rs

@@ -3285,14 +3285,14 @@ mod tests {
                         start: self.anchor_before(range.end)?,
                         end: self.anchor_before(range.start)?,
                         reversed: true,
-                        goal_column: None,
+                        goal: SelectionGoal::None,
                     });
                 } else {
                     selections.push(Selection {
                         start: self.anchor_after(range.start)?,
                         end: self.anchor_before(range.end)?,
                         reversed: false,
-                        goal_column: None,
+                        goal: SelectionGoal::None,
                     });
                 }
             }

zed/src/editor/buffer/selection.rs

@@ -12,12 +12,19 @@ use std::{cmp::Ordering, mem, ops::Range};
 pub type SelectionSetId = time::Lamport;
 pub type SelectionsVersion = usize;
 
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub enum SelectionGoal {
+    None,
+    Column(u32),
+    ColumnRange { start: u32, end: u32 },
+}
+
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct Selection {
     pub start: Anchor,
     pub end: Anchor,
     pub reversed: bool,
-    pub goal_column: Option<u32>,
+    pub goal: SelectionGoal,
 }
 
 impl Selection {