This PR is an attempt in fixing #1232 where unknown at-rules were hoisted outside of its parent instead of staying in place. This PR is not adding proper support for @apply, but is just used as an example.

I added some tests with various syntaxes as seen in https://drafts.csswg.org/css-mixins-1/#apply-rule. I also added some nesting to make sure that works as expected.

Added some tests with the following input:

.foo {
  @apply --mixin;
  @apply --mixin-no-args();
  @apply --mixin-args(1, 2);
  @apply --mixin-with-empty-body {};
  @apply --mixin-with-body {
    color: red;
  }
  .bar {
    @apply --nested;
  }
}

Before, this would generate:

@apply --mixin;

@apply --mixin-no-args();

@apply --mixin-args(1, 2);

@apply --mixin-with-empty-body {
  
}

@apply --mixin-with-body {
  color: red;
}

@apply --nested;

As you can see, the .foo and .bar are even gone, because at some point they won't contain any declarations anymore and are just removed.

But with this PR, it will generate:

.foo {
  @apply --mixin;
  @apply --mixin-no-args();
  @apply --mixin-args(1, 2);
  @apply --mixin-with-empty-body {
    
  }
  @apply --mixin-with-body {
    color: red;
  }
}

.foo .bar {
  @apply --nested;
}

Like I mentioned before, this doesn't add proper support for @apply, so this is just an attempt in properly printing the correct output. There is no validation related to the actual mixins that are being applied here.

Fixes: #1232