Matthew Schaefer is not just a boon to the Islanders, but to the NHL as a whole. The 18-year-old defenseman, unanimously named the Calder Trophy recipient Wednesday , has already been featured in some of the league’s marketing, making a video with Matt Martin’s twin girls to explain how the draft lottery works. If he continues on the path he set in his rookie season, it stands to reason Schaefer will soon be one of the faces of the league. And, certainly, he’ll be one of the faces of the 2027 All-Star Game at UBS Arena. “When you look at Matt Schaefer, you can look at what he’s accomplished on the ice, which is a record for an 18-year-old defenseman,” NHL commissioner Gary Bettman told The Post at the UJA-Federation of New York’s Sports Annual Luncheon on Thursday. “And you look at his maturity, his personality, the way he interacts with people.…