Add an abstraction of the gallery code in `media-editor.js` called...

Add an abstraction of the gallery code in `media-editor.js` called `wp.media.collection`. This will be the basis for parsing `[gallery]`-like shortcodes in the media editor (thing playlists, collections of PDFs, etc).

There are currently no instances of this. Those will be forthcoming.

See #26631.


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