refactor(core): Use /omit-if-no-ref/ for behaviors.

* Use lesser-known DT features to skip behaviors not referenced
  in the user keymap
* Update the behaviors to skip code if no nodes found.
* Remove some empty config/data structs where unused in
  behaviors.

app/src/behaviors/behavior_toggle_layer.c

@@ -15,6 +15,8 @@
 
 LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
 
+#if DT_HAS_COMPAT_STATUS_OKAY(DT_DRV_COMPAT)
+
 struct behavior_tog_config {};
 struct behavior_tog_data {};
 
@@ -44,3 +46,5 @@ static struct behavior_tog_data behavior_tog_data;
 DEVICE_AND_API_INIT(behavior_tog, DT_INST_LABEL(0), behavior_tog_init, &behavior_tog_data,
                     &behavior_tog_config, APPLICATION, CONFIG_KERNEL_INIT_PRIORITY_DEFAULT,
                     &behavior_tog_driver_api);
+
+#endif /* DT_HAS_COMPAT_STATUS_OKAY(DT_DRV_COMPAT) */