[1.1.0] nav_menu component : extra content in the case of mega menu

Parent is working because it's parent menu entry.
But the strict structure is not good and should accept render array.

We could add an items_megamenu as array prop to allow richer content. If is_mega_menu and empty items_megamenu, then fallback on items to avoid breaking.

Or we could have a new component.

Until then here is the story updated to reflect the megamenu.