Synthesis of Multifunctionalized 1,2,3,4-Tetrahydropyridines, 2,3-Dihydropyridin-4(1H)-ones, and Pyridines from Tandem Reactions Initiated by [5+1] Cycloaddition of N-Formylmethyl-Substituted Enamides to Isocyanides: Mechanistic Insight and Synthetic Application