The name “LevelEditor.MainMenu” is registered in C++ , you could find all the registered menu in ToolMenus.cpp .
https://docs.unrealengine.com/en-US/API/Developer/ToolMenus/UToolMenus/index.html
it had a private varaiable menus, contained all the related UToolMenu.
how to get all the menu name, you can try with the list_menu method I show above.