Copies Navigation Area infrastrucutre from lib/navigation.php in Gutenberg. This
allows a Navigation block to be associated with a particular area which persists
when switching theme.
Props antonvlasenko, mamaduka, spacedmonkey.
See #54337.
Built from https://develop.svn.wordpress.org/trunk@52145
git-svn-id: http://core.svn.wordpress.org/trunk@51737 1a063a9b-81f0-0310-95a4-ce76da25c4cd