Update SCSS rules around mixin location given impending Sass changes

[tylersticka]

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:

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

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

The output would be:

.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:

.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.