Kernel: Detect 5-button PS/2 mouse if present :^)

The detection works very similarly to how we detect a mouse wheel, just
another magical sequence of "set sample rate" requests to the mouse
followed by an ID check.

Kernel/MousePacket.h

@@ -37,6 +37,8 @@ struct MousePacket {
         LeftButton = 0x01,
         RightButton = 0x02,
         MiddleButton = 0x04,
+        BackButton = 0x08,
+        ForwardButton = 0x10,
     };
 
     unsigned char buttons { 0 };