Influence of Structure and Composition on Linear and Nonlinear Optical Properties of Clusters: A Case Study on Clusters (n-Bu4N)3[MS4M‘3XX‘3], MS4M‘3X(PPh3)3, and (n-Bu4N)2[MOS3M‘3X‘3] (M = Mo, W; M‘ = Cu, Ag; X = Cl, Br; X‘ = Cl, Br, I, SCN)