Ads for “Barbie” claimed it was for people who hate her and love her, which left me wondering: What about people who don’t care about her at all? Turns out the comedy/drama has us covered, too. The key to the movie is director/co-writer Greta Gerwig’s ability — previously demonstrated in “Lady Bird” and “Little Women” as well as in performances in “Maggie’s Plan” and “Frances Ha” — to make…