Menus: Bring back line break between menu items.

While removing line breaks was great for all use cases except justified menu
items, it broke menus with justified menu items.
Reverts [34321].

Props wp-architect.
Fixes #35107.


Built from https://develop.svn.wordpress.org/trunk@36081


git-svn-id: https://core.svn.wordpress.org/trunk@36046 1a063a9b-81f0-0310-95a4-ce76da25c4cd