One problem with modern horror is that it rarely has a sense of humor about itself—or if it does, the best it can do is point to its own self-awareness with a wink and a nudge, as if awkwardly apologizing for its own existence. Writer-director Jane Schoenbrun’s Teenage Sex and Death at Camp Miasma, premiering here at the Cannes Film Festival, breaks that cycle without breaking a sweat: it’s funny, charming, even breezy at times. And if there’s any justice in the world of movies—there isn’t always, but we can hope— Gillian Anderson’s performance will get the same kind of attention Demi Moore’s did a few years ago for the exhausting gross-out fest The Substance . What Anderson does here, as a 1990s scream queen turned glamorous, eccentric recluse, is sublime, and her co-star Hannah Einbinder (of Hacks ) keeps pace with her every minute. They’re having fun, and so are we: together, they make this movie a pleasure.…