Add support for ThumbHash for image placeholders

[JacobDB]

Feature Description

ThumbHash is an algorithm that generates a very low resolution approximation of an image in tiny file sizes. It'd be nice to be able to lazy load using this as the placeholder.

https://evanw.github.io/thumbhash/

[westonruter]

Essentially an improvement over the dominant color currently used in the Image Placeholders plugin? I seem to recall this or blurhash being discussed previously:

• Add low quality image placeholders using background color #19
• https://wordpress.slack.com/archives/C02KGN5K076/p1644944891681339
• https://wordpress.slack.com/archives/C02KGN5K076/p1662480224819099
• https://wordpress.slack.com/archives/C02KGN5K076/p1643732248572309?thread_ts=1643732248.572309&cid=C02KGN5K076
• https://wordpress.slack.com/archives/C02KGN5K076/p1648566998788159

In that last message, @pbearne says:

We found that blurhash wasn't a good fit and just creating a primary color was much better

I'm not sure why I wasn't a good fit at that time 2 years ago. Maybe this has changed? Maybe because https://github.com/kornrunner/php-blurhash requires PHP 7.3+? If that is the reason, maybe it could be a progressive enhancement over the dominant color if still on PHP 7.2. But eventually the PHP version will be bumped anyway.

[pbearne]

When we were looking at this all the other options needed some client-side JS or bloated the HTML
we didn't look at thumbhash

Looking at the code example we could look at offering this as an option with the dominant color plugin as it is all server-side
https://github.com/SRWieZ/thumbhash

[swissspidy]

FWIW, BlurHash can be turned into CSS gradients that look approximately the same. I'm doing that for https://github.com/swissspidy/media-experiments and the result is a nice improvement over just a dominant color. Also, everything is done client-side, so the PHP version requirement is not relevant.

ThumbHash does sound interesting though, I'm gonna try integrating that in Media Experiments as well and do some comparison with BlurHash.

The image placeholders plugin here is a bit unmaintained right now, so I'm not sure it's worth spending too much time on implementing at the moment.