Support leading type selector in nesting

[valtlai]

Firefox 117 supports the following:

.foo {
  mark {
    background: lime;
  }

  main & {
    font-size: 2em;
  }
}

(CodePen demo)

Other browsers don’t currently support a nested selector starting with a symbol, so the nesting selector & or the :is() pseudo-class is needed:

.foo {
  & mark {
    background: lime;
  }

  :is(main) & {
    font-size: 2em;
  }
}

The CSSWG removed this restriction, but the other browser aren’t yet implemented/shipped this change.

Lightning CSS should apply these transformations, so the CSS authors can use a bare type selector at the start of the entire selector.