Issue #3461691: Add a YAML Format

Closes #3461691

src/Plugin/menu_migration/Format/YamlFormat.php

+<?php
+
+namespace Drupal\menu_migration\Plugin\menu_migration\Format;
+
+use Drupal\Component\Serialization\Yaml;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\menu_migration\Attribute\MenuMigrationFormat;
+
+/**
+ * Provides the YAML import/export Format.
+ */
+#[MenuMigrationFormat(
+  id: 'yaml',
+  label: new TranslatableMarkup('YAML')
+)]
+class YamlFormat extends FormatBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function encode(array $menuTree) {
+    return Yaml::encode($menuTree);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function decode(mixed $menuTree) {
+    return Yaml::decode($menuTree);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function allowedExtensions() {
+    return ['yaml', 'yml'];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function defaultExtension() {
+    return 'yaml';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function mimeType() {
+    // @todo Not sure what the good mimetype is, using text for now.
+    return 'text/plain';
+  }
+
+}