Lewis-Acid Properties of Technetium(VII) Dioxide Trifluoride, TcO<sub>2</sub>F<sub>3</sub>: Characterization by <sup>19</sup>F, <sup>17</sup>O, and <sup>99</sup>Tc NMR Spectroscopy and Raman Spectroscopy, Density Functional Theory Calculations of TcO<sub>2</sub>F<sub>3</sub>, M<sup>+</sup>TcO<sub>2</sub>F<sub>4</sub><sup>-</sup> [M = Li, Cs, N(CH<sub>3</sub>)<sub>4</sub>], and TcO<sub>2</sub>F<sub>3</sub>·CH<sub>3</sub>CN, and X-ray Crystal Structure of Li<sup>+</sup>TcO<sub>2</sub>F<sub>4</sub><sup>-</sup>