Issue #3176908 by kostyashupenko, mherchel: Add variables to Olivero's...

 * @file

 * Theme override for a fieldset element and its children.

 *

 * Available variables:

 * - attributes: HTML attributes for the <fieldset> element.

 * - errors: (optional) Any errors for this <fieldset> element, may not be set.

 * - required: Boolean indicating whether the <fieldset> element is required.

 * - legend: The <legend> element containing the following properties:

 *   - title: Title of the <fieldset>, intended for use as the text

       of the <legend>.

 *   - attributes: HTML attributes to apply to the <legend> element.

 * - description: The description element containing the following properties:

 *   - content: The description content of the <fieldset>.

 *   - attributes: HTML attributes to apply to the description container.

 * - children: The rendered child elements of the <fieldset>.

 * - prefix: The content to add before the <fieldset> children.

 * - suffix: The content to add after the <fieldset> children.

 * - title_display: Title display setting.

 * - inline_items: Boolean indicating whether the <fieldset> items are inline.

 *

 * @see template_preprocess_fieldset()

 * @see olivero_preprocess_fieldset()

 */