Add edit button inside of nav menu items for posts/pages

[westonruter]

When a nav menu item is added for a post/page, there should be a button that allows you to edit the item in the Customizer.

This will depend on:

• Implement logic to fetch the post and postmeta settings for a given post ID to add a section without having to do a preview load. The pattern here has already been established in the new insertAutoDraftPost function which makes an Ajax request that returns a response containing all of the post/postmeta settings that would be needed for the new post section. (via List all posts of a given post type in the section #146 (comment))
• Listening for insertions of MenuItemControl instances and modifying the control to add the button when embedded.
• Potentially implement the breadcrumb functionality that was done for adding an Edit Menu button to the Custom Menu widget. See #32683.

[westonruter]

Closely related to #156.