core: Miniscule incremental progress on websocket message handling in C.

src/taskstub.js.h

@@ -82,4 +82,11 @@ void tf_taskstub_on_error(tf_taskstub_t* stub, JSValue error);
 */
 void tf_taskstub_on_print(tf_taskstub_t* stub, JSValue arguments);
 
+/**
+** Terminate a task stub.
+** @param stub The task stub to kill.
+** @return A promise if it is happening asynchronously.
+*/
+JSValue tf_taskstub_kill(tf_taskstub_t* stub);
+
 /** @} */