Navigation: Improve how non-link blocks appear in old nav-menus.php screen

[noisysocks]

If a user activates a theme that has add_theme_support( 'block-nav-menus' ), adds a non-link block to their Navigation, and then opens the old Menus screen (nav-menus.php), they will see the block they added as a Block menu item like so:

There's a few improvements we could make here:

• Prevent user from being able to nest menu items underneath a Block menu item, since in most cases this requires changing the HTML in the Block menu item.

• If the theme does not have add_theme_support( 'block-nav-menus' ), properly indicate that the menu item will not display on the frontend.

See #24503 (comment).

[noisysocks]

If the theme does not have add_theme_support( 'block-nav-menus' ), properly indicate that the menu item will not display on the frontend.

We'll need a simple design for this.

[Thelmachido]

Closing this issue due to the Navigation Editor project being moved to an inactive status on the feature projects page in coordination with the project leads. If this work is picked back up, issues can be revisited and reopened as needed.

The work will continue on the navigation block.