Pyridine-2(1H)-thione in Heterocyclic Synthesis: Synthesis of Some New Nicotinic Acid Ester, Thieno[2, 3-b]pyridine, Pyrido[3′, 2′: 4, 5]thieno [3, 2-d]pyrimidine, and Thiazolylpyrazolo[3, 4-b]pyridine Derivatives