Allow the sortable items selector in wpNavMenu to be overridden

[westonruter]

For Menu Customizer (#32576) we are trying to re-use as much of nav-menu.js as possible. However, there is a problem in wpNavMenu uses an overly-generic selector > * for the sortable items. We need to allow this to be overriddable in the Customizer to be .customize-control-nav_menu_item, but nav-menu.js doesn't currently expose the selector via its options property. This needs to change.

[westonruter]

Source (props adamsilverstein): ​https://github.com/xwp/wordpress-develop/commit/0f89312d238aa7ef80316d6d77633eaacf1e243f

[ocean90]

In 32745:

Nav menu: Allow the sortable items selector in wpNavMenu to be overridden.

props adamsilverstein.
fixes #32633.