Support reusable percentage templates in SCSS keyframes

Description

Outer keyframes need to accept permitted Sass statements such as @include, @content, loops, and conditionals. A mixin containing percentage steps must also parse correctly without depending on a later include.

@mixin frames {
  0% { opacity: 0 }
  100% { opacity: 1 }
}

@keyframes fade {
  @include frames;
}