Use extend to create menu-main.html.twig template

[thamas]

Extend menu.html.twig instead of duplicating it.

[tomphippen]

Is the solution to this one editing menu.html.twig to add .setAttribute('data-drupal-selector',menu_classes) to level 0 menus and remove menu-main.html.twig.

It's the only difference in files and I can't see that it would have any negative effects if other menus get a data attribute

[ckrina]

Closing in favour of D.O. issue: https://www.drupal.org/project/drupal/issues/2936880