Add auto-sized lazy images feature

[captainbrosset]

Adding sizes=auto to an image that also has loading=lazy is a really cool feature which makes it possible for the browser to reserve the right size for an image, even though it hasn't yet loaded it. This way, as you scroll a really long page that has many lazy loaded images, you don't get layout jumps.
This only works with lazy loaded images and, for now, is only on Chromium-based browsers.

See also:

• Auto-sizes for lazy-loaded images web-platform-tests/interop#761
• https://html.spec.whatwg.org/multipage/images.html#valdef-sizes-auto
• Add sizes=auto to lazy-loaded <img> whatwg/html#8008
• https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/img#auto_3

[ddbeck]

This looks promising to me. I've suggested some possible, but optional, description edits. Please merge, if you think it's ready. Thank you!

[ddbeck]

Oh, I forgot to mention. To match up with loading-lazy, I suggest changing the ID to sizes-auto.