Support `image-set` notation without `url` notation

[alexander-akait]

• Operating System: no matter
• Node Version: no matter
• NPM Version: no matter
• webpack Version: no matter
• css-loader Version: no matter

Feature Proposal

.class {
  background-image: image-set("img1x.png" 1x, "img2x.png" 2x);
}

Feature Use Case

Use require on this images.