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/editor/src/element.rs

@@ -8668,7 +8668,7 @@ impl LineWithInvisibles {
                     let fragment_end_x = fragment_start_x + shaped_line.width;
                     if x < fragment_end_x {
                         return Some(
-                            fragment_start_index + shaped_line.index_for_x(x - fragment_start_x),
+                            fragment_start_index + shaped_line.index_for_x(x - fragment_start_x)?,
                         );
                     }
                     fragment_start_x = fragment_end_x;