BY ROBERT TAYLOR THE SAGE OF Great Cranberry ISLAND M any years have passed since he saw his first angel perched in a wild pear tree. Never has he forgotten the intensity of the experience, the feelings it aroused in him, and the glory that shone around. "Every May since then the angels congregate," he says. He speaks with a shy smile because he doesn't expect you to take him literally. Angels shouldn't be handled like a sack of potatoes. For the vision- arv painter it is the vision, not the size of the wing-feathers,