fix(feature-targeting): prevent accidental nesting of mdc-feature-targets mixin

[mmalerba]

Nesting this mixin could lead to unexpected results. (e.g. the following property would not get emitted if the color styles were requested)

@include mdc-feature-targets($feat-structure) {
  @include mdc-feature-targets($feat-color) {
    color: red;
  }
}

This PR makes the above pattern an error by using a global variable to track when we're inside the mixin and raise an error if the mixin is included again in that context.

[kfranqueiro]

Thanks for this PR! I was definitely wary of this potential mistake, so throwing an error on it will be super useful.