Polyfill wc_is_running_from_async_action_scheduler().

[jeffstieler]

Supports WC versions < 4.0.0.

Fixes #5378.

This PR introduces a polyfill for the wc_is_running_from_async_action_scheduler() function for stores running WooCommerce < 4.0.0.

The call to wc_is_running_from_async_action_scheduler() was introduced in this PR. Curiously, the PHP 5.6 + WC 3.8 build did not fail because this code path does not get executed by any tests.

Detailed test instructions:

• On main
• Install WC 3.9.3 on a test store
• Load the WP dashboard
• Verify PHP Fatal error occurs
• Switch to this branch
• Load the WP dashboard
• Verify no PHP Fatal error occurs

Changelog Note:

Fix: use of WC 4.0+ function.

[octaedro]

Thank you @jeffstieler for this fix. Code looks good and I tried this on an ephemeral site and it tested well.
Good job

[octaedro]

@jeffstieler Probably it's not related to this issue, but using this branch on an ephemeral site I saw this Notice in the Home screen

Notice: register_rest_route was called incorrectly. The REST API route definition for wc/store/cart is missing the required permission_callback argument. For REST API routes that are intended to be public, use __return_true as the permission callback. Please see Debugging in WordPress for more information. (This message was added in version 5.5.0.) in /srv/users/user98f1e275/apps/user98f1e275/public/wp-includes/functions.php on line 5225

Notice: register_rest_route was called incorrectly. The REST API route definition for wc/store/cart/items/(?P[\w-]{32}) is missing the required permission_callback argument. For REST API routes that are intended to be public, use __return_true as the permission callback. Please see Debugging in WordPress for more information. (This message was added in version 5.5.0.) in /srv/users/user98f1e275/apps/user98f1e275/public/wp-includes/functions.php on line 5225

Notice: register_rest_route was called incorrectly. The REST API route definition for wc/store/products/(?P[\d]+) is missing the required permission_callback argument. For REST API routes that are intended to be public, use __return_true as the permission callback. Please see Debugging in WordPress for more information. (This message was added in version 5.5.0.) in /srv/users/user98f1e275/apps/user98f1e275/public/wp-includes/functions.php on line 5225

Notice: register_rest_route was called incorrectly. The REST API route definition for wc/store/products/collection-data is missing the required permission_callback argument. For REST API routes that are intended to be public, use __return_true as the permission callback. Please see Debugging in WordPress for more information. (This message was added in version 5.5.0.) in /srv/users/user98f1e275/apps/user98f1e275/public/wp-includes/functions.php on line 5225

[jeffstieler]

@octaedro those route definitions are in the Blocks repo. I believe these warnings stem from WP 5.1.