Media: introduce a "Heading" view for improved accessibility.

* Add a new media view: `wp.media.view.Heading` designed to add accessibility friendly headers in the media library/modal.
* Add an initial "Attachments list" heading above the attachments list.

Props afercia.
Fixes #36925.


git-svn-id: https://develop.svn.wordpress.org/trunk@44969 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/media.php

@@ -3762,6 +3762,9 @@ function wp_enqueue_media( $args = array() ) {
 		'updateVideoPlaylist'         => __( 'Update video playlist' ),
 		'addToVideoPlaylist'          => __( 'Add to video playlist' ),
 		'addToVideoPlaylistTitle'     => __( 'Add to Video Playlist' ),
+
+		// Headings
+		'attachmentsList'             => __( 'Attachments list' ),
 	);
 
 	/**