Add a mixin for `image-set()`

[robsonsobral]

.webp is already here and .jxl is coming. We need a good way to use image-set(), as it browser support is [fairly good, around 75%](https://caniuse.com/?search=image-set()).

What we need to cover:

• support for the following properties:
  • background-image;
  • content;
  • cursor;
• a default set of types and resolutions and that can be overridden;
• a prefered order of types:
  • image/jxl;
  • image/webp;
  • image/avif;
  • image/jpg;
  • image/png;
  • image/gif.
• the fallback declaration can be a jpeg, a png or a gif.

Questions

• How to deal with jpeg, the only type which doesn't support transparency, among the default list of types?
• How to deal with gif, the only type which doesn't support an alpha channel, but only matte color, among the default list of types?
• Should we ignore gif as a png can have a reduced color palette as well?