type JsPromiseContinuationCallback = Option<unsafe extern "system" fn(_: JsValueRef, _: *mut c_void)>;
A promise continuation callback.
The host can specify a promise continuation callback in JsSetPromiseContinuationCallback. If
a script creates a task to be run later, then the promise continuation callback will be called with
the task and the task should be put in a FIFO queue, to be run when the current script is
done executing.
The task, represented as a JavaScript function.
The data argument to be passed to the callback.