Autocomplete custom options missing from translation

Problem/Motivation

I'd like to use an autocomplete field with custom options in a webform that is translated.

While the managed list of options are translatable, "custom" options are not.

Steps to reproduce

1. Create a webform with an autocomplete field that uses custom options.
   
   e..g adding this element to the test_translation webform
   

   autocomplete_field_custom_options:
     '#type': autocomplete
     '#title': 'Autocomplete (custom)'
     '#format_items': comma
     '#autocomplete_items':
       - Seven
       - Eight
       - Nine
   

2. Navigate to the Translate tab
3. Edit a translation

Note that the custom options are not listed for translation.

Proposed resolution

Include autocomplete_items as a translatable property.

I think defining defineTranslatableProperties for webform\Element\WebformAutocomplete would do it.

  /**
   * {@inheritdoc}
   */
  protected function defineTranslatableProperties() {
    return array_merge(parent::defineTranslatableProperties(), ['autocomplete_items']);
  }

Remaining tasks

1. Create a patch
2. Write accompanying test

User interface changes

This would expose autocomplete custom options as translatable.

API changes

None.

Data model changes

None.