Jonathan Waxman's step-by-step guide to the compleat Thanksgiving dinner -- turkey and stuffing and all the sides -- in just two hours. Now, that's pilgrims' progress.   Many things in life can be done in a couple of hours. Cooking Thanksgiving dinner isn't one of them -- or so we thought until Washington Park chef Jonathan Waxman generously divulged the foolproof 120-minute technique he developed on a dare seventeen years ago and has been perfecting ever since. The avatar of California-casual cooking keeps things predictably simple with a fourteen-ingredient shopping list, and in his opinion, bigger is decidedly not better: "No one in New York has a huge kitchen," he says. "No one wants a huge turkey." The twelve-pounder his recipe calls for cooks in under an hour and a half. Which still leaves plenty of time to whip up oyster-and-apple stuffing, rich giblet gravy, crunchy Brussels sprouts and bacon, earthy roasted-carrot-and-endive salad, and homemade cranberry sauce, not to mention two kinds of potatoes, and spaghetti squash. Okay, so it doesn't include dessert. Isn't that what guests are for? Prepare Thanksgiving Dinner in 2 Hours>>>   The Menu Buffet-style Thanksgiving Serves 6 to 10. Roast turkey and gravy Oyster-apple stuffing Brussels sprouts with bacon Sweet potatoes Spaghetti squash Mashed potatoes Cranberry sauce Roasted-carrot-and-endive salad   The Wine