Multi-value form element

Provides a multi-value form element that wraps any number of form elements.
The wrapped elements will be repeated based on the cardinality specified on the wrapper.
It basically brings the delta cardinality concept of fields to form elements.
Each set of child elements ("delta") can be re-ordered with drag-and-drop.

$form['contacts'] = [
  '#type' => 'multivalue',
  '#title' => $this->t('Contacts'),
  '#cardinality' => MultiValue::CARDINALITY_UNLIMITED,
  'name' => [
    '#type' => 'textfield',
    '#title' => $this->t('Name'),
  ],
  'mail' => [
    '#type' => 'email',
    '#title' => $this->t('E-mail'),
  ],
];