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/platform.rs

@@ -792,6 +792,14 @@ impl PlatformInputHandler {
             cx,
         )
     }
+
+    #[allow(unused)]
+    pub fn character_index_for_point(&mut self, point: Point<Pixels>) -> Option<usize> {
+        self.cx
+            .update(|window, cx| self.handler.character_index_for_point(point, window, cx))
+            .ok()
+            .flatten()
+    }
 }
 
 /// A struct representing a selection in a text buffer, in UTF16 characters.
@@ -882,6 +890,16 @@ pub trait InputHandler: 'static {
         cx: &mut App,
     ) -> Option<Bounds<Pixels>>;
 
+    /// Get the character offset for the given point in terms of UTF16 characters
+    ///
+    /// Corresponds to [characterIndexForPoint:](https://developer.apple.com/documentation/appkit/nstextinputclient/characterindex(for:))
+    fn character_index_for_point(
+        &mut self,
+        point: Point<Pixels>,
+        window: &mut Window,
+        cx: &mut App,
+    ) -> Option<usize>;
+
     /// Allows a given input context to opt into getting raw key repeats instead of
     /// sending these to the platform.
     /// TODO: Ideally we should be able to set ApplePressAndHoldEnabled in NSUserDefaults