Converted plugin discovery

Closes #3521835

src/Attribute/Styleguide.php

+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\styleguide\Attribute;
+
+use Drupal\Component\Plugin\Attribute\Plugin;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+
+/**
+ * Defines a Styleguide attribute for plugin discovery.
+ *
+ * @see \Drupal\styleguide\StyleguideInterface
+ * @see \Drupal\styleguide\StyleguidePluginManager
+ * @see plugin_api
+ */
+#[\Attribute(\Attribute::TARGET_CLASS)]
+class Styleguide extends Plugin {
+
+  /**
+   * Constructs a Styleguide attribute.
+   *
+   * @param string $id
+   *   The plugin ID.
+   * @param \Drupal\Core\StringTranslation\TranslatableMarkup $label
+   *   The label of the plugin.
+   * @param \Drupal\Core\StringTranslation\TranslatableMarkup|null $description
+   *   (optional) A description of the plugin.
+   * @param string|null $deriver
+   *   (optional) The deriver class.
+   * @param string|null $provider
+   *   (optional) The machine name of the provider module.
+   */
+  public function __construct(
+    public readonly string $id,
+    public readonly TranslatableMarkup $label,
+    public readonly ?TranslatableMarkup $description = NULL,
+    public readonly ?string $deriver = NULL,
+    public ?string $provider = NULL,
+  ) {}
+
+}