Activation of Terminal Alkynes at the Sulfur-Rich Bimetallic Site [Mo III2 Cp 2 (μ-SMe) 3 ] + :  Alkyne−Vinylidene Conversion and C−S and C−C Couplings Promoted by Addition of Unsaturated Substrates (RC⋮CH, RN⋮C, SCS). Crystal Structures of μ-η 1 :η 2 -Vinylidene, μ-η 1 :η 2 -Acetylide, and μ-η 1 :η 3 -Vinyl−Thioether Compounds