Add Support for Dynamic Properties when Using PHP 8.1.28 or Greater

[ldr2273]

Describe the bug

When updating to PHP 8.1.28 or higher, deprecation notices regarding Dynamic Properties are displayed:

PHP Deprecated:  Creation of dynamic property Storefront_Product_Sharing::$plugin_url is deprecated in /srv/htdocs/wp-content/plugins/storefront-product-sharing/storefront-product-sharing.php on line 85
PHP Deprecated:  Creation of dynamic property Storefront_Product_Sharing::$plugin_path is deprecated in /srv/htdocs/wp-content/plugins/storefront-product-sharing/storefront-product-sharing.php on line 86

Isolating the problem (mark completed items with an [x]):

• I have deactivated other plugins and themes and confirmed this bug occurs when only WooCommerce + Storefront theme are active.
• I can reproduce this bug consistently using the steps below.

To Reproduce

1. Make sure debugging is true/enabled: https://developer.wordpress.org/advanced-administration/debug/debug-wordpress/
2. Ensure PHP version is 8.1.28 or higher
3. Install and activate the plugin
4. Check logs

Expected behavior

The plugin should be activated without any warnings.

Environment

WordPress 6.5.4
WooCommerce 8.9.3
PHP 8.2 (though it happens at 8.1.28 or higher)

Initially reported in 8254237-zen