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/linux/x11/window.rs

@@ -846,9 +846,9 @@ impl X11WindowStatePtr {
         if let PlatformInput::KeyDown(event) = input {
             let mut state = self.state.borrow_mut();
             if let Some(mut input_handler) = state.input_handler.take() {
-                if let Some(ime_key) = &event.keystroke.ime_key {
+                if let Some(key_char) = &event.keystroke.key_char {
                     drop(state);
-                    input_handler.replace_text_in_range(None, ime_key);
+                    input_handler.replace_text_in_range(None, key_char);
                     state = self.state.borrow_mut();
                 }
                 state.input_handler = Some(input_handler);