fix action pattern in useTransition / useOptimistic

[rickhanlonii]

When you create a function with the action suffix, it's because you know that it's being called inside of a transition. So it doesn't really make sense to immediately wrap the body of a function named sendMessageAction with startTransition.

The reason the useOptimistic example did this is because the child Thread component was not awaiting the action prop that was passed in, but the pattern should really be that you await the action prop if you expose it. That allows the action callback to be either synchronous or asynchronous without requiring an additional startTransition to wrap the await in the action.