"SpongeBob SquarePants”—a bright, silly, and sweet new party of a musical—is big-hearted and big fun; director and conceiver Tina Landau has skillfully adapted the popular Nickelodeon cartoon for the stage in a buoyant and exuberant production that is both entertaining and moving for theatregoers of all ages, offering a welcomed antidote to these beleaguered times.