Extend inside root block should error

[mgreter]

Continued from #1651

.a {
  display: block;
}

.b {
  @at-root (with: media) {
    @extend .a;
  }
}

Should error:

Error: Extend directives may only be used within rules.

Error is disabled for now until we figure out how to handle with and without correctly

[xzyfer]

IIRC problem is that throw this error the wrong time. It's impossible know if something can be extended to after eval is complete.

[mgreter]

Error throwing should be at the right position in the code by now.
I rather think we do not implement with/without in css bubbling.