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/markdown_preview/src/markdown_renderer.rs

@@ -206,7 +206,7 @@ fn render_markdown_list_item(
                 let secondary_modifier = Keystroke {
                     key: "".to_string(),
                     modifiers: Modifiers::secondary_key(),
-                    ime_key: None,
+                    key_char: None,
                 };
                 Tooltip::text(
                     format!("{}-click to toggle the checkbox", secondary_modifier),