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/include/linker/zmk-linker-defs.h

@@ -0,0 +1,6 @@
+
+
+#define→EVENT_TYPE_SECTIONS()→  →       →       →       \
+→       →       __event_type_start = .;→      →       \
+→       →       KEEP(*(".event_type_*"));→    →       \
+→       →       __event_type_end = .;→→       →       \