AXHg<sub>3</sub>P<sub>2</sub>S<sub>8</sub> (A <b>=</b> Rb, Cs; X <b>=</b> Cl, Br): New Excellent Infrared Nonlinear Optical Materials with Mixed‐Anion Chalcohalide Groups of Trigonal Planar [HgS<sub>2</sub>X]<sup>3−</sup> and Tetrahedral [HgS<sub>3</sub>X]<sup>5−</sup>