A Rare Bit of Welsh Cookery:It’s time for Sunday Supper at the end of a long holiday weekend. My inclination is to avoid heavy casseroles or roasts, and I land on page 112 of the “Joy of Cooking” where my eyes scan a familiar title – “Welsh Rarebit.”“Is it Rarebit, or Rabbit?” I ask aloud.It is perhaps one of the great culinary riddles of all time. And, Mrs. Rombaurer does little to illuminate