Customize: Deprecate nav menu classes that are no longer used, instead of...

Customize: Deprecate nav menu classes that are no longer used, instead of removing them immediately.

* Deprecate PHP classes `WP_Customize_New_Menu_Section` and `WP_Customize_New_Menu_Control`.
* Deprecate JS class `wp.customize.Menus.NewMenuControl`.
* Also introduce `wp.customize.Menus.createNavMenu()` for logic to create nav menus separately from the logic for handling UI interactions.

Amends [41768].
See #40104, #42364.
Fixes #42357.

Built from https://develop.svn.wordpress.org/trunk@42034


git-svn-id: https://core.svn.wordpress.org/trunk@41868 1a063a9b-81f0-0310-95a4-ce76da25c4cd