Synthesis, Linear, and Nonlinear Optical Properties of Phosphonato-Substituted Bithiophenes Derived from 2,2-Dimethyl-1,3-propanediol: Synthesis, Linear, and Nonlinear Optical Properties of Phosphonato-Substituted Bithiophenes Derived from 2,2-Dimethyl-1,3-propanediol