replace extend plugin with rework-inherit

replace extend plugin with rework-inherit. not sure if this is exactly what you wanted, but it's easier than maintaining a subset of another library. only downside is that you're downloading extra code since we disable the media query inheritance.

didn't add any additional tests as there are already tests in the library. however, all the current extend plugin issues are now solved.

i can add you as a maintainer if you'd like.