Naveen Kumar Chasing brunch to the brink of apocalypse is Stephen Sondheim at his most extreme, and the world premiere of “Here We Are,” which opened Off Broadway at the Shed on Sunday, is a study in overabundance. Appetite for the late composer’s final musical, written with David Ives and directed by Joe Mantello, has made it a hot ticket among those familiar enough with the moneyed types depicted onstage to potentially be the butt of the joke.