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/src/platform/keystroke.rs

@@ -34,6 +34,7 @@ impl Keystroke {
         {
             let ime_modifiers = Modifiers {
                 control: self.modifiers.control,
+                platform: self.modifiers.platform,
                 ..Default::default()
             };
 
@@ -124,6 +125,9 @@ impl Keystroke {
     /// Produces a representation of this key that Parse can understand.
     pub fn unparse(&self) -> String {
         let mut str = String::new();
+        if self.modifiers.function {
+            str.push_str("fn-");
+        }
         if self.modifiers.control {
             str.push_str("ctrl-");
         }