Should throw error when root selector has "&"

[nschonni]

In the Ruby Sass 3.2/3.3 the following will throw the error "Base-level rules cannot contain the parent-selector-referencing character '&'."

&.bar {
  color: red;
}

[mike-spainhower]

👍 Just ran into this.

[mike-spainhower]

Thanks @mgreter ❤️

[mgreter]

Well, we have one regression with this fix in #930, so I'm not yet sure if we can keep this in for 3.2!