function mymodule_preprocess_page(&$vars) {

  $route_name = Drupal::routeMatch()->getRouteName();

  // This would add a class based on the route.  You'd need the
  // pathauto module enabled.
  $vars['page_classes'][] = Drupal::service('pathauto.alias_cleaner')->cleanString($route_name);

  // This would add the classes asked for in the question.
  if ($route_name == 'entity.node.edit_form') {
    $vars['page_classes'][] = 'entity-edit';
  }
  elseif ($route_name == 'node.add') {
    $vars['page_classes'][] = 'entity-create';
  }
}