Chemistry of Gold in Molten Alkali Metal Polychalcophosphate Fluxes. Synthesis and Characterization of the Low‐Dimensional Compounds A3AuP2Se8 (A: K, Rb, Cs), A2Au2P2Se6 (A: K, Rb), A2AuPS4 (A: K, Rb, Cs) , and AAuP2S7 (A: K, Rb).