initial data is bad idea needs to rethink

[MiroslavPetrik]

The initial data can work with action similarly like initial state works with useReducer().
That means that it becomes the value/data even when the envelope state is in failure or invalid mode. Currently the types indicate that it would be null which is wrong.

Also it should be optional, currently its required on the Action component.

Finally, it should be separate from the success data, because both initial and success data could be const string types, which won't fit the current single generic slot.