mixins-before-declarations should not raise warning with top-level mixins

[BPScott]

Currently @includeing a mixin at the top level (i.e. not inside a selector) incorrectly raises a 'mixins-before-delcarations' warning, when this should be acceptable (there are no properties before it!).

To reproduce, add the following to the bottom of mixins-before-declarations.scss:

@include hello;

@media (min-width: 50em) {
  @include hello;
}

@supports (display: flex) {
  @include hello;
}

@at-root {
  @include hello;
}

I would expect that this would raise no additional linting errors.

[shaneog]

👍