Use bundled scripts for polyfills

[swissspidy]

In WordPress 5.0 we now have wp_get_script_polyfill() and the wp-polyfill script handle.

wp_default_packages_vendor() does register some polyfills by default when enqueuing wp-polyfill: wp-polyfill-fetch, wp-polyfill-node-contains, wp-polyfill-formdata, wp-polyfill-element-closest. wp-polyfill itself contains polyfills for things like Promises.

In this plugin, we manually add polyfills for #Element.closest and window.fetch, as well as Promises.

To make things easier for us, it would make sense to use the polyfills provided by WordPress for this. Makes maintenance a bit easier again. For WordPress < 5.0 we could still use the current approach.