template_preprocess_select(&$variables)
Prepares variables for select element templates.
Default template: select.html.twig.
It is possible to group options together; to do this, change the format of $options to an associative array in which the keys are group labels, and the values are associative arrays in the normal $options format.
Parameters
$variables: An associative array containing:
element: An associative array containing the properties of the element. Properties used: #title, #v