Disable "block-nav-menus" feature for the purposes of removing the "experimental" status on the Navigation Editor

[anton-vlasenko]

Description

For the purposes of removing the "experimental" status on the Navigation Editor, we agreed we would disable the theme opt-in from the screen until core developers have had enough time to improve the feature. That means no additional blocks.
Fixes #34265.

Steps to test this PR.

1. Enable support for the block-nav-menus feature.
   If you theme doesn't support this feature you can "enable" it programmatically by modifying the plugins/gutenberg/lib/navigation-page.php file.
   To do so go to that file and add this line

get_theme_support( 'block-nav-menus' );

at the very beginning of the gutenberg_navigation_init's function body (around line 31).
2. Go to the Gutenberg ->Navigation Editor -> Experiments page.
3. Enable the Enable Navigation screen setting, save the changes.

4. Go to the Gutenberg -> Navigation page.
5. Create a new menu.
6. Click on the "Start empty" link.

7. Click on the plus sign.
Expected result:
Make sure you only see "regular" menu items like those on the screenshot below:

You shouldn't see block menu items, like Social Links block on the screenshot below:

Types of changes

New feature / Breaking change.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[anton-vlasenko]

I'm not sure if we can use the @see tag in inline comments (I mean not in the PHPDOC block).
Is there a better way to document these changes?

[talldan]

Thanks @anton-vlasenko! LGTM

I'm not sure if we can use the @see tag in inline comments (I mean not in the PHPDOC block).
Is there a better way to document these changes?

This seems fine to me. I'm not sure whether documentation generation will pick them up, but these inline docs won't be picked up anyway.

[anton-vlasenko]

Thank you for the review, @talldan !