Rename ime_key -> key_char and update behavior (#20953)

As part of the recent changes to keyboard support, ime_key is no longer
populated by the IME; but instead by the keyboard.

As part of #20877 I changed some code to assume that falling back to key
was
ok, but this was not ok; instead we need to populate this more similarly
to how
it was done before #20336.

The alternative fix could be to instead of simulating these events in
our own
code to push a fake native event back to the platform input handler.

Closes #ISSUE

Release Notes:

- Fixed a bug where tapping `shift` coudl type "shift" if you had a
binding on "shift shift"

crates/gpui/src/platform/mac/window.rs

@@ -1283,18 +1283,17 @@ extern "C" fn handle_key_event(this: &Object, native_event: id, key_equivalent:
     }
 
     if event.is_held {
-        let handled = with_input_handler(&this, |input_handler| {
-            if !input_handler.apple_press_and_hold_enabled() {
-                input_handler.replace_text_in_range(
-                    None,
-                    &event.keystroke.ime_key.unwrap_or(event.keystroke.key),
-                );
+        if let Some(key_char) = event.keystroke.key_char.as_ref() {
+            let handled = with_input_handler(&this, |input_handler| {
+                if !input_handler.apple_press_and_hold_enabled() {
+                    input_handler.replace_text_in_range(None, &key_char);
+                    return YES;
+                }
+                NO
+            });
+            if handled == Some(YES) {
                 return YES;
             }
-            NO
-        });
-        if handled == Some(YES) {
-            return YES;
         }
     }
 
@@ -1437,7 +1436,7 @@ extern "C" fn cancel_operation(this: &Object, _sel: Sel, _sender: id) {
     let keystroke = Keystroke {
         modifiers: Default::default(),
         key: ".".into(),
-        ime_key: None,
+        key_char: None,
     };
     let event = PlatformInput::KeyDown(KeyDownEvent {
         keystroke: keystroke.clone(),