feat: Split physical layout selection sync.

* Ensure the split peripherals have the same selected physical
  layout on connection and change.

app/include/zmk/physical_layouts.h

@@ -8,6 +8,13 @@
 
 #include <zephyr/kernel.h>
 #include <zmk/matrix_transform.h>
+#include <zmk/event_manager.h>
+
+struct zmk_physical_layout_selection_changed {
+    uint8_t selection;
+};
+
+ZMK_EVENT_DECLARE(zmk_physical_layout_selection_changed);
 
 struct zmk_key_physical_attrs {
     int16_t width;