Initial event manager work, and two first events.

* Add initial event manager implementation,
  roughly mimicking Nordic's API.
* Add `position_state_changed` and
  `keycode_state_changed` events.
* Hook up HID and keymap to new events
  instead of using behaviour global event
  crazy.

app/src/events/keycode_state_changed.c

@@ -0,0 +1,6 @@
+
+
+#include <kernel.h>
+#include <zmk/events/keycode-state-changed.h>
+
+ZMK_EVENT_IMPL(keycode_state_changed);