Issue #2944552 by mpdonadio, bhanuprakashnani, liquidcms, Sophie.SK,...

 * Provides a form element for date selection.

 *

 * Properties:

 * - #default_value: An array with the keys: 'year', 'month', and 'day'.

 *   Defaults to the current date if no value is supplied.

 * - #default_value: A string for the default date in 'Y-m-d' format.

 * - #size: The size of the input element in characters.

 *

 * @code

 * $form['expiration'] = array(

 * $form['expiration'] = [

 *   '#type' => 'date',

 *   '#title' => $this->t('Content expiration'),

 *   '#default_value' => array('year' => 2020, 'month' => 2, 'day' => 15,)

 * );

 *   '#default_value' => 2020-02-05',

 * ];

 * @endcode

 *

 * @FormElement("date")