/**
 * Implements hook_form_FORM_ID_alter().
 */
function seven_form_node_page_edit_form_alter(&$form, \Drupal\Core\Form\FormStateInterface &$form_state, $form_id) {
  $form['field_slideshow']['#states'] = [
    'visible' => [
      'select[name="field_selector"]' => ['value' => 'slideshow']
    ]
  ];
  $form['field_image']['#states'] = [
    'visible' => [
      'select[name="field_selector"]' => ['value' => 'image']
    ]
  ];
  $form['field_video_embed']['#states'] = [
    'visible' => [
      'select[name="field_selector"]' => ['value' => 'videoembed']
    ]
  ];
}