feat(behaviors): Add key position to key events

Extended the virtual key position system from combos so that each sensor
also gets a virtual key position. This allows sensor behaviors to use
the behavior queue API.

app/include/zmk/virtual_key_position.h

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2022 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <zmk/matrix.h>
+#include <zmk/sensors.h>
+
+/**
+ * Gets the virtual key position to use for the sensor with the given index.
+ */
+#define ZMK_VIRTUAL_KEY_POSITION_SENSOR(index) (ZMK_KEYMAP_LEN + (index))
+
+/**
+ * Gets the virtual key position to use for the combo with the given index.
+ */
+#define ZMK_VIRTUAL_KEY_POSITION_COMBO(index) (ZMK_KEYMAP_LEN + ZMK_KEYMAP_SENSORS_LEN + (index))