Deadkeys 2 (#20612)

Re-land of #20515 with less brokenness

In particular it turns out that for control, the .characters() method
returns the control code. This mostly didn't make a difference, except
when the control code matched tab/enter/escape (for
ctrl-y,ctrl-[/ctrl-c) as we interpreted the key incorrectly.

Secondly, we were setting IME key too aggressively. This led to (in vim
mode) cmd-shift-{ being interpreted as [, so vim would wait for a second
[ before letting you change tab.

Release Notes:

- N/A

crates/gpui/examples/input.rs

@@ -580,9 +580,9 @@ impl Render for InputExample {
             .children(self.recent_keystrokes.iter().rev().map(|ks| {
                 format!(
                     "{:} {}",
-                    ks,
+                    ks.unparse(),
                     if let Some(ime_key) = ks.ime_key.as_ref() {
-                        format!("-> {}", ime_key)
+                        format!("-> {:?}", ime_key)
                     } else {
                         "".to_owned()
                     }