The new dramedy “Eighth Grade,” about a painfully shy 13-year old stumbling through her last week of middle school, is the first film for writer-director Bo Burnham and for its young star, Elsie Fisher. But there’s more to the movie than the usual teen angst and acne. There’s the loneliness that, research tells us, is becoming more pervasive for young people as social media increases in influence. Tracy Smith talks with Burnham, and with San Diego State professor Jean Twenge, author of “iGen,” about how the internet is making adolescence even tougher.