Issue #3113746 by justcaldwell: Support List (text) fields

src/Plugin/Field/FieldFormatter/ListStringListClassFormatter.php

+<?php
+
+namespace Drupal\element_class_formatter\Plugin\Field\FieldFormatter;
+
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\Field\FormatterBase;
+use Drupal\Core\Template\Attribute;
+
+/**
+ * Formatter for displaying list (text) in an HTML list.
+ *
+ * @FieldFormatter(
+ *   id="list_string_list_class",
+ *   label="List (with class)",
+ *   field_types={
+ *     "list_string",
+ *   }
+ * )
+ */
+class ListStringListClassFormatter extends FormatterBase {
+
+  use ElementListClassTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function viewElements(FieldItemListInterface $items, $langcode) {
+    $elements = [];
+    $attributes = new Attribute();
+    $class = $this->getSetting('class');
+    if (!empty($class)) {
+      $attributes->addClass($class);
+    }
+
+    $field_storage = $items->getFieldDefinition()->getFieldStorageDefinition();
+    $options = options_allowed_values($field_storage);
+    foreach ($items as $delta => $item) {
+      $elements[$delta] = $options[$item->getValue()['value']];
+    }
+
+    return [
+      [
+        '#theme' => 'item_list',
+        '#items' => $elements,
+        '#list_type' => $this->getSetting('list_type'),
+        '#attributes' => $attributes->toArray(),
+      ],
+    ];
+  }
+
+}