Jack Thorne’s listless, shapeless, and dreadful new play “Sunday” centers on a book group of Gen Z-ers grappling with ennui; yes, it is as painful as that sounds.  Mr. Thorne doesn’t have much to say for this unconvincingly invented milieu, other than to wallow in the imagined ennui and dullness of a supposedly disillusioned generation to which he does not belong.