macos: detect IME input source change as part of keyDown event

Fixes #4539

AquaSKK is a Japanese IME (Input Method Editor) for macOS. It uses
keyboard inputs to switch between input modes. I don't know any other
IMEs that do this, but it's possible that there are others. Prior to
this change, the keyboard inputs to switch between input modes were
being sent to the terminal, resulting in erroneous characters being
written.

This change adds a check during keyDown events to see if the input
source changed _during the event_. If it did, we assume an IME captured
it and we don't pass the event to the terminal.

This makes AquaSKK functional in Ghostty.

macos/Sources/Helpers/KeyboardLayout.swift

@@ -0,0 +1,14 @@
+import Carbon
+
+class KeyboardLayout {
+    /// Return a string ID of the current keyboard input source.
+    static var id: String? {
+        if let source = TISCopyCurrentKeyboardInputSource()?.takeRetainedValue(),
+           let sourceIdPointer = TISGetInputSourceProperty(source, kTISPropertyInputSourceID) {
+            let sourceId = unsafeBitCast(sourceIdPointer, to: CFString.self)
+            return sourceId as String
+        }
+
+        return nil
+    }
+}