Issue #3317493 by Charlie ChX Negyesi: Integrate into core menus

group_content_menu.info.yml

group_content_menu.install

group_content_menu.module

group_content_menu.services.yml

group_content_submenu/README.md

...all/field.storage.group_content_menu.parent_menu_link.yml

...all/field.storage.group_content_menu.parent_menu_name.yml

group_content_submenu/group_content_submenu.module

group_content_submenu/group_content_submenu.services.yml

group_content_submenu/src/GroupContentSubmenuAlter.php

group_content_submenu/src/GroupContentSubmenuInsert.php

group_content_submenu/src/MenuLinkManager.php

group_content_submenu/src/ProxyMenulink.php

group_content_submenu/src/ShadowlinkHelper.php

modules/group_content_submenu/group_content_submenu.info.yml

...nk_reference/config/schema/menu_link_reference.schema.yml

modules/menu_link_reference/menu_link_reference.info.yml

...ence/src/Plugin/Field/FieldType/MenuLinkReferenceItem.php

...erence/src/Plugin/Field/FieldWidget/MenuLinkReference.php

src/Controller/GroupContentMenuController.php