Issue #3280023 Added "Lowercase plugin" and unit test.

Closes #3280023

src/Plugin/token_modifier/Lowercase.php

+<?php
+
+namespace Drupal\token_modifier\Plugin\token_modifier;
+
+use Drupal\token_modifier\Plugin\TokenModifierPluginBase;
+
+/**
+ * Makes all characters of the token lowercase.
+ *
+ * @TokenModifier(
+ *   id = "lowercase",
+ *   name = @Translation("Lowercase"),
+ *   description = @Translation("Lowercase the returned string.")
+ * )
+ */
+class Lowercase extends TokenModifierPluginBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transform(string $text, array $data = [], array $options = []) {
+    return strtolower($this->token->replace($text, $data, $options));
+  }
+
+}