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/gpui/examples/input.rs

@@ -178,7 +178,7 @@ impl TextInput {
         if position.y > bounds.bottom() {
             return self.content.len();
         }
-        line.index_for_x(position.x - bounds.left())
+        line.closest_index_for_x(position.x - bounds.left())
     }
 
     fn select_to(&mut self, offset: usize, cx: &mut Context<Self>) {
@@ -380,7 +380,7 @@ impl EntityInputHandler for TextInput {
         let last_layout = self.last_layout.as_ref()?;
 
         assert_eq!(last_layout.text, self.content);
-        let utf8_index = last_layout.index_for_x(point.x - line_point.x);
+        let utf8_index = last_layout.index_for_x(point.x - line_point.x)?;
         Some(self.offset_to_utf16(utf8_index))
     }
 }