Correct other end visual block functionality (#27678)

Closes #27385

Builds on #27604 so that `vim::OtherEnd` works in visual block mode.
This is accomplished by reversing the order of active selections in the
buffer when the user hit `o`, so that the cursor moves diagonally across
the selection. The current behavior is preserved for `shift-o`, which is
how the cursors behave in vim.

We'll close #27604 since this encapsulates that change, but if you'd
prefer to take only the visual block motion component, we'll keep the
branch for #27604 open.

Test case: growing a box down and to the right, other ending, followed
by growing and shrinking the box:


https://github.com/user-attachments/assets/1df544e1-efce-4354-b354-bbfec007a7df

Test case: growing a box up and to the left, other ending, followed by
growing and shrinking the box:


https://github.com/user-attachments/assets/2f6d7729-c63a-4486-960b-23474c2e507a



Release Notes:
- Improved visual block mode when cursor is at beginning of selection
- Improved visual block mode so that `o` and `shift-o` reach parity with
vim

---------

Co-authored-by: KyleBarton <kjbarton4@gmail.com>

crates/editor/src/selections_collection.rs

@@ -655,6 +655,25 @@ impl<'a> MutableSelectionsCollection<'a> {
             .collect();
         self.select(selections);
     }
+    pub fn reverse_selections(&mut self) {
+        let map = &self.display_map();
+        let mut new_selections: Vec<Selection<Point>> = Vec::new();
+        let disjoint = self.disjoint.clone();
+        for selection in disjoint
+            .iter()
+            .sorted_by(|first, second| Ord::cmp(&second.id, &first.id))
+            .collect::<Vec<&Selection<Anchor>>>()
+        {
+            new_selections.push(Selection {
+                id: self.new_selection_id(),
+                start: selection.start.to_display_point(map).to_point(map),
+                end: selection.end.to_display_point(map).to_point(map),
+                reversed: selection.reversed,
+                goal: selection.goal,
+            });
+        }
+        self.select(new_selections);
+    }
 
     pub fn move_with(
         &mut self,