LibWeb: Define a helper constant for the platform ctrl-ish key

For all intents and purposes, the cmd (super) key on macOS should be
treated as the ctrl key.

(cherry picked from commit 71b25e3e5a70937f1e2031b94541c04a68d28efa;
amended to fix minor conflict)

Kernel/API/KeyCode.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/EnumBits.h>
+#include <AK/Platform.h>
 #include <AK/Types.h>
 
 #define ENUMERATE_KEY_CODES                                    \
@@ -166,6 +167,12 @@ enum KeyModifier {
     Mod_Mask = Mod_Alt | Mod_Ctrl | Mod_Shift | Mod_Super | Mod_AltGr | Mod_Keypad,
 
     Is_Press = 0x80,
+
+#if defined(AK_OS_MACOS)
+    Mod_PlatformCtrl = Mod_Super,
+#else
+    Mod_PlatformCtrl = Mod_Ctrl,
+#endif
 };
 
 AK_ENUM_BITWISE_OPERATORS(KeyModifier);