Customize: Draw attention to "Add Items" button after creating a new menu.

Introduce `wp.customize.utils.highlightButton()` and `wp.customize.Menus.MenuSection#highlightNewItemButton()`.

Props bpayton, melchoyce, afercia, westonruter.
Fixes #42114.

Built from https://develop.svn.wordpress.org/trunk@41930


git-svn-id: https://core.svn.wordpress.org/trunk@41764 1a063a9b-81f0-0310-95a4-ce76da25c4cd