Provide alter hook for the build of the PatternLayout

Closes #3441987

modules/ui_patterns_layouts/src/Plugin/Layout/PatternLayout.php

@@ -94,6 +94,8 @@ class PatternLayout extends LayoutDefault implements PluginFormInterface, Contai
       $fields[$region_name] = $regions[$region_name];
     }
 
+    $this->moduleHandler->alter('ui_patterns_layouts_display', $configuration, $this, $regions);
+
     return [
       '#type' => 'pattern',
       '#id' => $this->getPluginDefinition()->get('additional')['pattern'],