Jean Shepherd, the humorist, author and personality in the broadcasting corridors of radio and television, insists that he hates nostalgia. All right, for argument's sake, let's buy that. But his "The Phantom of the Open Hearth," which can be seen on public television's, "Visions" series tonight at 9 o'clock is a wonderfully offbeat reminiscence of a young man enduring the maturation trials of a junior prom in the industrial Middle West of the 1940's...