Add support for @apply rule to apply mixins

[QtKaii]

Closing this PR as the functionality is already supported in Less.js without needing an @apply rule. As matthew-dean pointed out, mixins can be used directly like:

.mixin() {
  color: blue;
  font-size: 12px;
}

.test {
  .mixin();
}

No need for @apply syntax since this is already part of the language.

[QtKaii]

Tests pass :)

[matthew-dean]

This code change doesn't have anything to do with #3675, which has to do with being more forgiving about evaluating at-rules when assigned to a custom property. Using @apply to use a mixin is not necessary; it's already supported by the language.