Implement suitable fallback for Nav block on front end of site when no menu selected

[getdave]

Description

In #36721 we learnt that the Nav block doesn't currently have a suitable fallback on the front end of the site when a menu is not selected.

This PR implements in the simplest way possible by falling back to rendering a core/page-list block with it's default attributes.

As Nav is already set up to handle displaying this block the styles all work perfectly out of the box.

if we want to limit the number of items shown then we'll need to modify core/page-list to have a maxPages prop so we can control the number of items. We might be able to provide this via context. I suggest we do this in a separate PR. Update: I added a commit which introduces a maxPages attribute to the Nav block. We can then pass this to the Page List block via context to control the number of items that are displayed.

Addresses part of #36721

How has this been tested?

Before testing use Fakerpress Plugin (or similar) to add a load of pages to your site. Some should be nested parent/child.

1. Open Site Editor.
2. Remove any existing Nav block.
3. Add Nav block from scratch. Don't add any items!
4. Save your empty Nav block and publish the post.
5. Go to front of site.
6. See Page List rendered by default even though no Menu has been selected.
7. Return to Editor and add a menu for your nav block.
8. Save and return to front of site.
9. Check that your menu is rendered.

Screenshots

Types of changes

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).

[getdave]

Some additional feedback here #36721 (comment)

[getdave]

As a followup I'm going to implement finding a Nav Menu. I have the code ready once this is merged.

Update: it's ready at #36740