Polly Stenham's That Face has been hailed as a great debut, but a playwright's first professional production is rarely really their first workPolly Stenham wrote That Face when she was 19. Photograph: Alex Macnaughton / Rex FeaturesWith the long-awaited West End transfer of Polly Stenham's Royal Court debut That Face, our thoughts turn to first plays and the recent boom in young writers. Even by recent standards, Ste…