Feature: input processor behavior invocation (#2714)

refactor(pointing): Allow stopping event propagation

    Allow input processors to return a special value if a given input event
    should not be further processed/propagated.

feat(pointing): Add behavior input processor

    Add the ability to intercept certain input events and trigger behaviors
    when they occur.

Co-authored-by: Jorge Villalobos <minusfive@users.noreply.github.com>
Co-authored-by: Cem Aksoylar <caksoylar@users.noreply.github.com>

app/include/zmk/combos.h

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <zephyr/devicetree.h>
+
+#define ZMK_COMBOS_UTIL_ONE(n) 1
+
+#define ZMK_COMBOS_LEN                                                                             \
+    COND_CODE_1(                                                                                   \
+        DT_HAS_COMPAT_STATUS_OKAY(zmk_combos),                                                     \
+        (DT_FOREACH_CHILD_STATUS_OKAY_SEP(DT_INST(0, zmk_combos), ZMK_COMBOS_UTIL_ONE, (+))), (0))