fix(split): Add define for local source.

* Add `ZMK_POSITION_STATE_CHANGE_SOURCE_LOCAL` and use
  it consinstently to fix bug w/ local `&reset`, `&bootloader`, etc.

app/include/zmk/events/position_state_changed.h

@@ -9,6 +9,8 @@
 #include <zephyr.h>
 #include <zmk/event_manager.h>
 
+#define ZMK_POSITION_STATE_CHANGE_SOURCE_LOCAL UINT8_MAX
+
 struct zmk_position_state_changed {
     uint8_t source;
     uint32_t position;