Port the Markup element to GraphQL 4.

Closes #3419973

src/Enum/WebformElementDisplayOn.php

+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\graphql_webform\Enum;
+
+/**
+ * Enumerates the possible display targets for webform elements.
+ *
+ * @see \Drupal\webform\Plugin\WebformElementDisplayOnInterface
+ */
+enum WebformElementDisplayOn: string {
+
+  case DISPLAY_ON_BOTH = 'both';
+  case DISPLAY_ON_FORM = 'form';
+  case DISPLAY_ON_VIEW = 'view';
+  case DISPLAY_ON_NONE = 'none';
+
+  /**
+   * Returns the enum values as an associative array.
+   *
+   * @return array<string, string>
+   *   The enum values as an associative array.
+   */
+  public static function toArray(): array {
+    $cases = [];
+    foreach (self::cases() as $case) {
+      $cases[$case->name] = $case->value;
+    }
+    return $cases;
+  }
+
+}