Adds "Subscription Search" feature

[ecaron]

Description of the Change

As discussed in 10up/ElasticPress#2867, and then submitted in 10up/ElasticPress#3029, this adds WooCommerce Subscription Search functionality to the WordPress admin section.

Note: This requires the hooks and filters introduced in 10up/ElasticPress#3029 to be functional

It also makes a small tweak to the class auto-loading to support developers using Windows, as well as bumped the composer dependencies to be in sync with the ElasticPress 4.2 libraries (necessary for PHP 8 support via phpunit).

How to test the Change

1. Install any plugin that leverages WooCommerce subscription-core
2. Add a subscription
3. Within the search interface, do a search with & without ElasticPress enabled - the search results will be the same (albeit ElasticPress' results will return much faster)

Changelog Entry

Added - New lab feature to support searching the shop_subscription post type (identical to shop_order in all other ways)
Changed - Adjusted the method used to determine if classes are loaded to work on Mac, Linux & Windows

Credits

Props @ecaron, @felipeelia

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[ecaron]

@felipeelia This has been updated also as you suggested in 10up/ElasticPress#3029

[ecaron]

Please let me know if you'd like the lint.yml updated to pass - looks like it could updated to sync with ElasticPress' lint.yml (in particular since chekalsky/phpcs-action was deprecated).