Add a dev-mode warning to `createSlice` to error if `initialState` is undefined

[markerikson]

I've seen some folks misspell initialState, and thus end up with no initial state.

This leads to combineReducers doing the "Reducer x returned undefined blah blah" error, but it's not as helpful as it could be.

We should throw a dev-mode error inside of createSlice itself if the initial state is undefined.

[dannielss]

I would like to solve this.

[dannielss]

My first contribution #2461

[markerikson]

That's merged, so I'll close this.