Implement character index for point (#23989)

Fixes #22939
Fixes #23970
Supersedes https://github.com/zed-industries/zed/pull/23469

Release Notes:

- Fixed a bug where Zed could crash with certain input sources on macOS

---------

Co-authored-by: Louis Brunner <louis.brunner.fr@gmail.com>
Co-authored-by: ben <ben@zed.dev>

crates/gpui/src/input.rs

@@ -62,6 +62,14 @@ pub trait EntityInputHandler: 'static + Sized {
         window: &mut Window,
         cx: &mut Context<Self>,
     ) -> Option<Bounds<Pixels>>;
+
+    /// See [`InputHandler::character_index_for_point`] for details
+    fn character_index_for_point(
+        &mut self,
+        point: crate::Point<Pixels>,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+    ) -> Option<usize>;
 }
 
 /// The canonical implementation of [`PlatformInputHandler`]. Call [`Window::handle_input`]
@@ -158,4 +166,15 @@ impl<V: EntityInputHandler> InputHandler for ElementInputHandler<V> {
             view.bounds_for_range(range_utf16, self.element_bounds, window, cx)
         })
     }
+
+    fn character_index_for_point(
+        &mut self,
+        point: crate::Point<Pixels>,
+        window: &mut Window,
+        cx: &mut App,
+    ) -> Option<usize> {
+        self.view.update(cx, |view, cx| {
+            view.character_index_for_point(point, window, cx)
+        })
+    }
 }