Update Tachyon to handle WebP images

[roborourke]

Sharp.js, the library used by Tachyon to dynamically resize images now has support for WebP, SVG, PDF, AVIF and GIFs, including animated GIFs.

We should at a minimum enable dynamic resizing of WebP now that WordPress 5.8 supports uploading that format.

The current pass through behaviour for animated GIFs and SVG images is fine, there's nothing much to gain there right now.

[rmccue]

We should at a minimum enable dynamic resizing of WebP now that WordPress 5.8 supports uploading that format.

We probably also need the reverse of what we have, transcoding WebP back to PNG for browsers that don't support it.