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/mac/platform.rs

@@ -1448,13 +1448,27 @@ unsafe fn ns_url_to_path(url: id) -> Result<PathBuf> {
 
 #[link(name = "Carbon", kind = "framework")]
 extern "C" {
-    fn TISCopyCurrentKeyboardLayoutInputSource() -> *mut Object;
-    fn TISGetInputSourceProperty(
+    pub(super) fn TISCopyCurrentKeyboardLayoutInputSource() -> *mut Object;
+    pub(super) fn TISGetInputSourceProperty(
         inputSource: *mut Object,
         propertyKey: *const c_void,
     ) -> *mut Object;
 
-    pub static kTISPropertyInputSourceID: CFStringRef;
+    pub(super) fn UCKeyTranslate(
+        keyLayoutPtr: *const ::std::os::raw::c_void,
+        virtualKeyCode: u16,
+        keyAction: u16,
+        modifierKeyState: u32,
+        keyboardType: u32,
+        keyTranslateOptions: u32,
+        deadKeyState: *mut u32,
+        maxStringLength: usize,
+        actualStringLength: *mut usize,
+        unicodeString: *mut u16,
+    ) -> u32;
+    pub(super) fn LMGetKbdType() -> u16;
+    pub(super) static kTISPropertyUnicodeKeyLayoutData: CFStringRef;
+    pub(super) static kTISPropertyInputSourceID: CFStringRef;
 }
 
 mod security {