Move the loader WP & WC version checks to the framework

Right now our plugin loader checks for the required PHP, WordPress, and WooCommerce versions before loading the plugin itself: https://github.com/skyverge/wc-plugin-framework/blob/release-v5/woocommerce/woocommerce-framework-plugin-loader-sample.php#L119-L121

We could try and move the WP & WC version checks to the plugin base class itself to help keep the Loader class as minimal as possible. Also consider moving all of the dependency logic to its own handler class to clean things up event further.