Creating Enum for the User mail constants

Closes #2582951

core/lib/Drupal/Core/Utility/UserEmailNotification.php

+<?php
+// phpcs:ignoreFile
+
+declare(strict_types=1);
+
+namespace Drupal\Core\Utility;
+
+/**
+ * Enumeration of constant related User email notification.
+ */
+enum UserEmailNotification: string {
+  /**
+   * Email notification operation for the welcome message sent to the user
+   * created by an administrator.
+   */  
+  case RegisterAdminCreated = 'register_admin_created';
+
+  /**
+   * Email notification operation for the welcome message sent to the user who
+   * registered and didn't need to be approved by an administrator.
+   */
+  case RegisterNoApprovalRequired = 'register_no_approval_required';
+
+  /**
+   * Email notification operation for the welcome message sent to the user who
+   * registered, but needs to be approved by an administrator.
+   */
+  case RegisterPendingApproval = 'register_pending_approval';
+
+  /**
+   * Email notification operation for the message sent to an administrator, after
+   * a user has registered and needs approval.
+   */
+  case RegisterPendingApprovalAdmin = 'register_pending_approval_admin';
+
+  /**
+   * Email notification operation for the message sent to the user who requested
+   * a password recovery.
+   */
+  case PasswordReset = 'password_reset';
+
+  /**
+   * Email notification operation for the message sent to the user whose status
+   * has been activated.
+   */
+  case StatusActivated = 'status_activated';
+
+  /**
+   * Email notification operation for the message sent to the user whose status
+   * has been set to blocked.
+   */
+  case StatusBlocked = 'status_blocked';
+
+  /**
+   * Email notification operation for the message sent to the user whose status
+   * has been set to canceled.
+   */
+  case StatusCanceled = 'status_canceled';
+
+  /**
+   * Email notification operation for the message sent to the user to confirm
+   * the account cancellation request.
+   */
+  case CancelConfirm = 'cancel_confirm';
+
+}