macOS: Improve deadkeys (#20515)

Closes #19738

This change refactors how we handle input on macOS to avoid simulating
our own IME. This fixes a number of small edge-cases, and also lets us
remove a bunch of code that had been added to work around bugs in the
previous version.

Release Notes:

- On macOS: Keyboard shortcuts are now handled before activating the IME
system, this enables using vim's default mode on keyboards that use IME
menus (like Japanese).
- On macOS: Improvements to handling of dead-keys. For example when
typing `""` on a Brazillian keyboard, you now get a committed " and a
new marked ", as happens in other apps. Also, you can now type cmd-^ on
an AZERTY keyboard for indent; and ^ on a QWERTZ keyboard now goes to
the beginning of line in vim normal mode, or `d i "` no requires no
space to delete within quotes on Brazilian keyboards (though `d f "
space` is still required as `f` relies on the input handler, not a
binding).
- On macOS: In the terminal pane, holding down a key will now repeat
that key (as happens in iTerm2) instead of opening the character
selector.

crates/gpui/src/platform/keystroke.rs

@@ -124,6 +124,9 @@ 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-");
         }