Allow using element inside modifier

[bgilhome-lds]

In some backend systems, a modifier is applied to the block and not an element. In this case, it would be useful to be able to write:

@block foo {
  @modifier bar {
    @element baz {
      background: red;
    }
  }
}

To compile to:

.foo--bar .foo__baz {
  background: red;
}

The PR achieves this, but might be better as an option (apply modifier to block or element).

[tbremer]

Thanks for the PR! Just wanted to acknowledge that I have seen it. I need to look at it a bit more closely though. Work is a bit busy, but hopefully within the next couple of days.