Add support for Media RSS / MailChimp

[sun]

core-standards currently uses the <enclosure> element, which is part of the RSS2 standard protocol:
https://validator.w3.org/feed/docs/rss2.html#ltenclosuregtSubelementOfLtitemgt

The <enclosure> tag is generated here:

wordpress-core-standards/src/Feed.php

Lines 39 to 41 in c7063e6

	?>
	<enclosure url="<?= esc_url($url) ?>" length="<?= esc_attr($filesize) ?>" type="<?= esc_attr($mime_type) ?>" />
	<?php

Example:

<enclosure url="https://example.com/wp-content/uploads/2020/03/KSC-860x573.jpg" length="54309" type="image/jpeg" />

As the RSS feed is often used in MailChimp for automated newsletters, we should output the information a second time and directly include the Media RSS tag as well:

Quick summary for using <media:content>: https://stackoverflow.com/a/48549541/811306
Spec: http://www.rssboard.org/media-rss#media-content
Long explanation: https://www.wired.com/2010/02/use_media_rss/