Isocyanide‐Based Three‐Component Synthesis of Highly Substituted 1,6‐Dihydro‐6,6‐dimethylpyrazine‐2,3‐dicarbonitrile, 3,4‐Dihydrobenzo[g]quinoxalin‐2‐amine, and 3,4‐Dihydro‐3,3‐dimethyl‐quinoxalin‐2‐amine Derivatives