TIVERTON, R.I. and FREETOWN, Mass. — Crimson in color and tart on the tongue, cranberries add both sustenance and symbolism to the Thanksgiving table. As with turkey, pumpkin and other hallmarks of America’s feast day, their historical significance stems from their role in fortifying early New Englanders against the harsh winters once common in this region. But although cranberries remain a Thanksgiving staple, the cold winters that gave rise to this custom are fast becoming a thing of yesteryear.