refactor(core): Extra event payloads to own types, refactor API.

* Make it easier to use *just* event payloads by defining the data,
  and then having event manager macros generate "wrapper structs"
* Improve is_*/cast_* APIs to hide details of full event struct.
* Create `zmk_event_t` typedef to pass to event handlers.
* Bring event names inline w/ consistent `zmk_` prefix.

app/src/events/activity_state_changed.c

@@ -7,4 +7,4 @@
 #include <kernel.h>
 #include <zmk/events/activity_state_changed.h>
 
-ZMK_EVENT_IMPL(activity_state_changed);
+ZMK_EVENT_IMPL(zmk_activity_state_changed);