Implement Helix Support (WIP) (#19175)

Closes #4642 

- Added the ability to switch to helix normal mode, with an additional
helix visual mode.
- <kbd>ctrl</kbd><kbd>h</kbd> from Insert mode goes to Helix Normal
mode. <kbd> i </kbd> and <kbd> a </kbd> to go back.
- Need to find a way to perform the helix normal mode selection with
<kbd> w </kbd>, <kbd>e </kbd>, <kbd> b </kbd> as a first step. Need to
figure out how the mode will interoperate with the VIM mode as the new
additions are in the same crate.

crates/text/src/selection.rs

@@ -84,6 +84,31 @@ impl<T: Copy + Ord> Selection<T> {
         }
         self.goal = new_goal;
     }
+
+    pub fn set_tail(&mut self, tail: T, new_goal: SelectionGoal) {
+        if tail.cmp(&self.head()) <= Ordering::Equal {
+            if self.reversed {
+                self.end = self.start;
+                self.reversed = false;
+            }
+            self.start = tail;
+        } else {
+            if !self.reversed {
+                self.start = self.end;
+                self.reversed = true;
+            }
+            self.end = tail;
+        }
+        self.goal = new_goal;
+    }
+
+    pub fn swap_head_tail(&mut self) {
+        if self.reversed {
+            self.reversed = false;
+        } else {
+            std::mem::swap(&mut self.start, &mut self.end);
+        }
+    }
 }
 
 impl<T: Copy> Selection<T> {