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/vim/src/motion.rs

@@ -2639,7 +2639,8 @@ fn find_backward(
     }
 }
 
-fn is_character_match(target: char, other: char, smartcase: bool) -> bool {
+/// Returns true if one char is equal to the other or its uppercase variant (if smartcase is true).
+pub fn is_character_match(target: char, other: char, smartcase: bool) -> bool {
     if smartcase {
         if target.is_uppercase() {
             target == other