Automatic entity form mode

This is a small helper module that makes it easier to use custom form modes.

By default, when creating a custom form mode, you can't use them without writing a hook in a custom module:

function my_module_entity_type_build(array &$entity_types) {
  $default_entity_form_class = $entity_types['node']->getFormClass('default');
  $entity_types['node']->setFormClass('my_custom_form_mode', $default_entity_form_class);
}

Without this hook, using \Drupal::service('entity.form_builder')->getForm($node, 'my_custom_form_mode'); will result in the following error:

Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException: The "node" entity type did not specify a "my_custom_form_mode" form class.

This module automatically registers all custom form modes so you can actually use them, without writing that extra hook.