Navigation Component: Add ul Automatically Where Needed

[Copons]

NavigationMenu can contains multiple lists or NavigationGroups, so it doesn't wrap its children with an ul.
On the other hand, NavigationItem is a li, so it needs to live inside an ul.
See the story for an example.

Possible solutions:

• By traversing the NavigationMenu children, we can determine if it needs an ul and automatically add it, instead of offloading the burden to the consumer.
  Depends on Navigation Component: Store the Navigation Tree in the Context #25246 exploration.

• Make NavigationMenu always render as an ul, and NavigationGroup as a <li><ul /></li>.
  In other words, if a menu contains only one list, it will be a simple lists; otherwise it will be a list of lists (whereas right now is a "series" of lists).

See #25057 (comment)