Fix extensions undefined (#123)

Co-authored-by: Naufan <[email protected]>

Author: nprizal

src/fake_definition.ts

@@ -271,7 +271,11 @@ export function buildWithFakeDefinitions(
       if (isObjectType(type) || isInterfaceType(type)) {
         for (const field of Object.values(type.getFields())) {
           const isExtensionField = field.astNode?.loc?.source === extensionSDL;
-          (field.extensions as any)['isExtensionField'] = isExtensionField;
+          if (field.extensions) {
+            (field.extensions as any)['isExtensionField'] = isExtensionField;
+          } else {
+            field.extensions = { isExtensionField };
+          }
         }
       }
     }