[Core] Tri Layer Keys (#19795)

Co-authored-by: wilba <wilba@wilba.tech>
Co-authored-by: Pablo Martínez <58857054+elpekenin@users.noreply.github.com>
Co-authored-by: Joel Challis <git@zvecr.com>
Co-authored-by: Nick Brassel <nick@tzarc.org>

quantum/process_keycode/process_tri_layer.h

@@ -0,0 +1,16 @@
+// Copyright 2023 QMK
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include "action.h"
+
+/**
+ * @brief Handles tri layer behavior
+ *
+ * @param keycode the keycode
+ * @param record the key record structure
+ * @return true continue handling keycodes
+ * @return false stop handling keycodes
+ */
+bool process_tri_layer(uint16_t keycode, keyrecord_t *record);