Fix Flow types of useEffectEvent

[eps1lon]

Summary

Just copied the types over from the internal types. Type error was hidden by overly broad FlowFixMe. With $FlowFixMe[not-a-function] we would've seen the actual issue:

Cannot return `dispatcher.useEffectEvent(...)` because  `T` [1] is incompatible with  undefined [2].Flow(incompatible-return)

How did you test this change?

• yarn flow dom-node
• CI