Efficient Synthesis of Dihydrofurans with Sulfide Groups by Ceric(IV) Ammonium Nitrate‐Mediated Oxidative Cycloaddition of 1,3‐Dicarbonyl Compounds to Vinyl Sulfides. Application to the Synthesis of Benzo[b]naphtho[2,3‐d]furan‐6,11‐dione and First Total Synthesis of Millettocalyxins C and Pongamol Methyl Ether.