vim: Fix increment panicking due to invalid utf8 offsets (#43101)

Fixes ZED-3ER

Release Notes:

- Fixed a panic when using vim increment on a multibyte character

crates/vim/src/normal/increment.rs

@@ -210,12 +210,12 @@ fn find_target(
         .map_or(false, |ch| ch.is_ascii_hexdigit());
     let mut pre_char = String::new();
 
+    let next_offset = offset
+        + snapshot
+            .chars_at(start_offset)
+            .next()
+            .map_or(0, |ch| ch.len_utf8());
     // Backward scan to find the start of the number, but stop at start_offset
-    let next_offset = if offset < snapshot.len() {
-        offset + 1usize
-    } else {
-        offset
-    };
     for ch in snapshot.reversed_chars_at(next_offset) {
         // Search boundaries
         if offset.0 == 0 || ch.is_whitespace() || (need_range && offset <= start_offset) {
@@ -823,6 +823,14 @@ mod test {
         cx.set_state("trueˇ 1 2 3", Mode::Normal);
         cx.simulate_keystrokes("ctrl-a");
         cx.assert_state("true ˇ2 2 3", Mode::Normal);
+
+        cx.set_state("falseˇ", Mode::Normal);
+        cx.simulate_keystrokes("ctrl-a");
+        cx.assert_state("truˇe", Mode::Normal);
+
+        cx.set_state("⚡️ˇ⚡️", Mode::Normal);
+        cx.simulate_keystrokes("ctrl-a");
+        cx.assert_state("⚡️ˇ⚡️", Mode::Normal);
     }
 
     #[gpui::test]