Fixes #92

Please check one:

• New tests created for this change
• Tests updated for this change

This PR:

• Adds new API
• Extends existing API, backwards-compatible
• Introduces a breaking change
• Fixes a bug

The previous solution I had wasnt checking that the at-word was directly next to the " :". So in the wikimedia file the :last-child was passing the condition for the @import statement even though theyre not next to each other.

@import 'mediawiki.mixins';

li {
  &:not( :last-child ) {
    margin-bottom: 16px;
  }
}

I added another line that checks that the at-word and " :" have nothing else between them. And I believe that @page is the only at-rule that can have a colon directly after it.