Update the stylelint rules to make sure they follow best practices

[ArekJanik]

• Disallow functions like: color: color.adjust(#6b717f, $lightness: -20%);
• Allow math.div
• Detect shorthand properties, like background
• Sort the block declaration like other declarations. Currently the background: { will be below all normal declarations as it is detected as a rule, not a declaration, eg:

padding: 0;
text-align: center;
background: {
  image: url("/assets/images/oops.jpg");
  position: center;
}

• Selector & should throw an error, as the & is not needed when followed by a space (often mistakenly used as & >)