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/terminal/src/mappings/keys.rs

@@ -343,7 +343,7 @@ mod test {
                 function: false,
             },
             key: "🖖🏻".to_string(), //2 char string
-            ime_key: None,
+            key_char: None,
         };
         assert_eq!(to_esc_str(&ks, &TermMode::NONE, false), None);
     }