Add extensions to GraphQLError

https://github.com/graphql/graphql-js/blob/master/src/error/GraphQLError.js#L89

types/graphql/error/GraphQLError.d.ts

@@ -56,6 +56,11 @@ export class GraphQLError extends Error {
    * The original error thrown from a field resolver during execution.
    */
   originalError?: Error;
+    
+  /**
+   * Extension fields to add to the formatted error.
+   */
+  extensions?: { [key: string]: any } | undefined;
 
   constructor(
     message: string,
@@ -64,5 +69,6 @@ export class GraphQLError extends Error {
     positions?: number[],
     path?: Array<string | number>,
     originalError?: Error,
+    extensions?: { [key: string]: any },
   );
 }