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/platform.rs

@@ -742,14 +742,14 @@ impl Keystroke {
             }
         }
 
-        // Ignore control characters (and DEL) for the purposes of ime_key
-        let ime_key =
+        // Ignore control characters (and DEL) for the purposes of key_char
+        let key_char =
             (key_utf32 >= 32 && key_utf32 != 127 && !key_utf8.is_empty()).then_some(key_utf8);
 
         Keystroke {
             modifiers,
             key,
-            ime_key,
+            key_char,
         }
     }