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.rs

@@ -683,6 +683,11 @@ impl PlatformInputHandler {
             .flatten()
     }
 
+    #[allow(dead_code)]
+    fn apple_press_and_hold_enabled(&mut self) -> bool {
+        self.handler.apple_press_and_hold_enabled()
+    }
+
     pub(crate) fn dispatch_input(&mut self, input: &str, cx: &mut WindowContext) {
         self.handler.replace_text_in_range(None, input, cx);
     }
@@ -780,6 +785,15 @@ pub trait InputHandler: 'static {
         range_utf16: Range<usize>,
         cx: &mut WindowContext,
     ) -> Option<Bounds<Pixels>>;
+
+    /// Allows a given input context to opt into getting raw key repeats instead of
+    /// sending these to the platform.
+    /// TODO: Ideally we should be able to set ApplePressAndHoldEnabled in NSUserDefaults
+    /// (which is how iTerm does it) but it doesn't seem to work for me.
+    #[allow(dead_code)]
+    fn apple_press_and_hold_enabled(&mut self) -> bool {
+        true
+    }
 }
 
 /// The variables that can be configured when creating a new window