NEW YORK —The fictional worlds of Charles Dickens and Sir Arthur Conan Doyle lovingly collide in playwright Mark Shanahan’s not-at-all humbug, “A Sherlock Carol” at New World Stages. This delightful mash-up features beloved literary characters, one from the Dickens’ novella “A Christmas Carol” and various elements from Doyle’s Sherlock Holmes stories—in particular, the only Holmes tale Doyle set during Christmas: “The…