Revert "gpui: Unify the index_for_x methods (#42162)" (#42505)

This reverts commit 082b80ec89.

This broke clicking, e.g. in snippets like

```rs
let x = vec![
    1, 2, //
    3,
];
```

clicking between `2` and `,` is quite off now.

Release Notes:

- N/A

crates/vim/src/visual.rs

@@ -371,10 +371,12 @@ impl Vim {
 
             loop {
                 let laid_out_line = map.layout_row(row, &text_layout_details);
-                let start =
-                    DisplayPoint::new(row, laid_out_line.index_for_x(positions.start) as u32);
+                let start = DisplayPoint::new(
+                    row,
+                    laid_out_line.closest_index_for_x(positions.start) as u32,
+                );
                 let mut end =
-                    DisplayPoint::new(row, laid_out_line.index_for_x(positions.end) as u32);
+                    DisplayPoint::new(row, laid_out_line.closest_index_for_x(positions.end) as u32);
                 if end <= start {
                     if start.column() == map.line_len(start.row()) {
                         end = start;