Navigation Block: Allow nav items to display horizontally on mobile

[kjellr]

On mobile screens, nav block items default to being displayed as a vertical list:

There's currently no way to override this and let the items appear in an inline horizontal list:

If a site only has a few nav items, it's quite possible the mobile nav will look better displayed this way, and that the "enable responsive menu" setting would be overkill. We should figure out a way for folks to opt-in to this. Maybe a "stack on mobile" option?

[jasmussen]

This is definitely a regression, and was not intentional. Let me take a look.

[jasmussen]

Fix in #32081.