Room‐Temperature Solid‐State Transformation of Na<sub>4</sub>SnS<sub>4</sub> ⋅ 14H<sub>2</sub>O into Na<sub>4</sub>Sn<sub>2</sub>S<sub>6</sub> ⋅ 5H<sub>2</sub>O: An Unusual Epitaxial Reaction Including Bond Formation, Mass Transport, and Ionic Conductivity