Add function to check if a post is a predefined page

Introduce `um_post_is_predefined_page` to determine if a given post ID corresponds to a predefined page. This enhances functionality by allowing checks against predefined page slugs mapped in the plugin configuration.

includes/um-core-functions.php

@@ -20,6 +20,27 @@ function um_predefined_page_slug_exists( $slug ) {
 	return array_key_exists( $slug, $predefined_pages );
 }
 
+/**
+ * @param int $post_id
+ *
+ * @return bool
+ */
+function um_post_is_predefined_page( $post_id ) {
+	$predefined_pages = UM()->config()->get( 'predefined_pages' );
+	$predefined_pages = array_keys( $predefined_pages );
+	if ( empty( $predefined_pages ) ) {
+		return false;
+	}
+
+	foreach ( $predefined_pages as $slug ) {
+		if ( um_is_predefined_page( $slug, $post_id ) ) {
+			return true;
+		}
+	}
+
+	return false;
+}
+
 /**
  * @param string $slug
  *