feature(keymaps): Keymap layer state event, highest layer API

* Fire an event w/ layer number, state, and timestamp
  when a layer is activated/deactivated.
* Add `zmk_keymap_highest_layer_active()` query.

Co-authored-by: KemoNine <mcrosson@kemonine.info>

app/src/events/layer_state_changed.c

@@ -0,0 +1,10 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <kernel.h>
+#include <zmk/events/layer-state-changed.h>
+
+ZMK_EVENT_IMPL(layer_state_changed);