fix: properly type extensions in GraphQLFormattedError

Following typescript documentation, it's not possible override interface property, we can only add new props.

Since it's declared as unknown dict, even if we merge `GraphQLErrorExtensions`, we can't access to our extensions without workaround and verbose typescript in source-code.

It's annoying since apollo expose `GraphQLFormattedError` instead `GraphQLError`

Refs: https://www.typescriptlang.org/docs/handbook/declaration-merging.html
Refs: apollographql/apollo-client#11789

Author: tpoisseau

src/error/GraphQLError.ts

@@ -275,7 +275,7 @@ export interface GraphQLFormattedError {
    * Reserved for implementors to extend the protocol however they see fit,
    * and hence there are no additional restrictions on its contents.
    */
-  readonly extensions?: { [key: string]: unknown };
+  readonly extensions?: GraphQLErrorExtensions;
 }
 
 /**