Support converting animated GIFs to WEBP

[alexrichterxyz]

I have some large GIF animations that I would like to post to my website. Currently, when trying to convert animated GIFs to WEBP, HUGO only processes the first frame (which is unexpected behaviour, perhaps a warning message would be appropriate?).

I think being able to retain animations when converting from GIF to WEBP would be a very useful feature.

[bep]

perhaps a warning message would be appropriate?).

Yea, maybe. But you do see the end result in the browser.

Yes, we should fix this. We should also support the case where the source is webp, which is tied into golang/go#53364

But we can start with this one, I guess, which we control ourself (https://github.com/bep/gowebp).