Synthesis of Dispiro‐[Indoline‐3,1′‐Cyclopropane‐2′,3′′‐Indoline]‐2,2′′‐Diones via Thermal [2+1] Cycloaddition Between 3‐Alkylidene‐7‐Aza‐2‐Indolones and 3‐Diazo‐7‐Aza‐2‐Oxindoles