Fix/nested mixins reorder

[pciarach]

Fixes: #1297

This is an attempt to preserve the same order of style declarations in the case of nested mixins.

I think that the previous assumption that the mixin after rules or at-rules will resolve to more rules or at-rules is mostly valid because of the CSS nesting module draft (https://drafts.csswg.org/css-nesting/#mixing). However, this draft does not cover non-standard mixin rules and thus should be excluded from the grouping logic.

[romainmenke]

@pciarach,
Thank you for this patch!

I've made a few edits because the original function was quite brittle.
It think my latest change is more in line with what I originally intended and I hope it also fixes all your issues.

I didn't see any new coverage from the mixin-nested-rules-after-media test, so I removed this. Please let me know if I overlooked something here.

[pciarach]

Thank you @romainmenke

It looks really nice. I've also added a minor change to improve readability a bit.

When it comes to the additional test - you are right, the similar thing is covered by other mixin tests.

[romainmenke]

Thank you for checking this @pciarach,

As a general note, it is best to not nit changes in someone else's project for readability because readability is highly subjective. The code I wrote was in the most readable form for me and since I be maintaining it that is more important.

I do appreciate the intention behind the changes 🙇

Do you mind changing it back?

[pciarach]

Sure, as you said you will be maintaining it so you have the last word 😉

[romainmenke]

Thank you so much for this @pciarach,
This is a complex feature and the test cases and pull request really helped to make it better 🎉

[pciarach]

@romainmenke Thanks for the help and such a quick responses!