Add support for CSS Nesting

[freshp86]

See https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_nesting.

It currently seems to not work and nested CSS rules are stripped from the output. For example passing the following

.parent {
 & .child {
  color: blue;
 }
}

produces an empty output. Same happens if the & nesting selector is omitted.

[laurentpayot]

Any update? CSS nesting is now considered a standard practice: https://caniuse.com/css-nesting