THE Biltmore Theater had better be paid up on its flood insurance. "Rabbit Hole," the wrenching new play by David Lindsay-Abaire that opened there last night, inspires such copious weeping among its audience that you wonder early on if you should have taken a life jacket. Do your best, though, to keep your eyes clear. Otherwise, you might miss some of the most revealingly nuanced acting to be seen on a stage or screen this year.