Add process_keycode handlers for new RGB Matrix and Underglow keycodes (#23896)

quantum/process_keycode/process_rgb_matrix.h

@@ -0,0 +1,10 @@
+// Copyright 2024 QMK
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include <stdint.h>
+#include <stdbool.h>
+#include "action.h"
+
+bool process_rgb_matrix(uint16_t keycode, keyrecord_t *record);