helix: Change f and t motions (#35216)

In vim and zed (vim and helix modes) typing "tx" will jump before the
next `x`, but typing it again won't do anything. But in helix the cursor
just jumps before the `x` after that. I added that in helix mode.
This also solves another small issue where the selection doesn't include
the first `x` after typing "fx" twice. And similarly after typing "Fx"
or "Tx" the selection should include the character that the motion
startet on.

Release Notes:

- helix: Fixed inconsistencies in the "f" and "t" motions

crates/text/src/selection.rs

@@ -104,6 +104,19 @@ impl<T: Copy + Ord> Selection<T> {
         self.goal = new_goal;
     }
 
+    pub fn set_head_tail(&mut self, head: T, tail: T, new_goal: SelectionGoal) {
+        if head < tail {
+            self.reversed = true;
+            self.start = head;
+            self.end = tail;
+        } else {
+            self.reversed = false;
+            self.start = tail;
+            self.end = head;
+        }
+        self.goal = new_goal;
+    }
+
     pub fn swap_head_tail(&mut self) {
         if self.reversed {
             self.reversed = false;