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/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 {