Update SCSS rules around mixin location given impending Sass changes

Currently, our lint definition enforces `@include` functions being at the beginning of a style block. This was a fine rule to settle on, because Sass would always output nested blocks _after_ non-nested properties.

So given this code:

```scss
@mixin vertical-rhythm($space: 1lh) {
> * + * {
margin-block-start: $space;
}
}

.example {
@include vertical-rhythm;
color: red;
}
```

The output would be:

```css
.example {
color: red;
}

.example > * + * {
margin-block-start: 1lh;
}
```

But Sass has announced they'll be changing this behavior in a future update. At some point, nested blocks will respect the output order: https://sass-lang.com/documentation/breaking-changes/mixed-decls/

With that change, the order of the output will be reversed:

```css
.example > * + * {
margin-block-start: 1lh;
}

.example {
color: red;
}
```

Because of that, I think we should consider either:

1. Enforcing `@include` statements and nesting to come _after_ top-level declarations. This would enforce current behavior.
2. Remove any `@include` order enforcement at all, allowing authors to leverage order as they see fit.