Feature/enable nav logic for all post types

[william-man]

Description of changes

This PR updates the chapter navigation to support all post types instead of being hardcoded to long-read. This is done by using post_type => 'any' in the get_posts query, which allows the plugin to work across all post types.

To test

1. Install this plugin into a site repo and fetch this branch feature/enable-nav-logic-for-all-post-types.
2. In this branch, copy the example template here into your site repo templates folder.
3. In admin, activate the plugin and check the Long read post type is available, and create a new long read post with some heading blocks at various levels. (Might need to flush permalinks in Settings > Permalinks save).
4. View the post, check that the long read navigation still renders on the page.
5. To check the navigation functionality is working for other post types, add this or similar to your template:

/**
 * Template Name: Long Read
 * Template Post Type: post, page, guidance, long-read
 */

6. On your chosen post type, create a new post similar to the one you created for the long-read post type and check that it renders the navigation functionality.
7. Check that it is not made available for other post types.

Checklist

• Changelog updated
• If new release: major version tag to be bumped after release (see docs)

[sbrody]

Works without error on ukss