Beneath a dim barroom light, Jon Styles walks onto the makeshift stage and takes the microphone. As the singer eases into Live’s 1994 hit “I Alone,” the crowd hoots and hollers with approval. “It’s easier not to be wise/You measure these things by your brains,” he begins with restrained intensity before unleashing the howling chorus. It’s Friday night and karaoke at the Tiburon Tavern is one of the hottest tickets in town, thanks, in great part, to Styles, an extraordinary singer who delivers performances you’d expect to hear at the S.B. Bowl rather than at a neighborhood bar on upper State Street.