Add the raw platform specific key code to SDL_Keysym

This allows applications to handle keys that SDL doesn't recognize, in a platform dependent way.

Fixes https://github.com/libsdl-org/SDL/issues/6390

include/SDL3/SDL_keyboard.h

@@ -68,7 +68,7 @@ typedef struct SDL_Keysym
     SDL_Scancode scancode;      /**< SDL physical key code - see SDL_Scancode for details */
     SDL_Keycode sym;            /**< SDL virtual key code - see SDL_Keycode for details */
     SDL_Keymod mod;             /**< current key modifiers */
-    Uint16 unused;
+    Uint16 raw;                 /**< The platform dependent scancode for this event */
 } SDL_Keysym;
 
 /* Function prototypes */