Enforce ASCII character encoding on string items

Closes #3120547

core/lib/Drupal/Core/Field/Plugin/Field/FieldType/StringItem.php

@@ -67,6 +67,9 @@ public function getConstraints() {
           '@max' => $max_length,
         ]);
       }
+      if ($this->getFieldDefinition()->getSetting('is_ascii') === TRUE) {
+        $options['charset'] = 'ASCII';
+      }
       $constraints[] = $constraint_manager->create('ComplexData', [
         'value' => [
           'Length' => $options,