Investigate supporting `combineSlices` natively in `configureStore`

[EskiMojo14]

combineSlices makes it very convenient to just do combineSlices(sliceA, sliceB), because it automatically mounts slices under their reducerPath. It would be very convenient to also be able to do something like configureStore({ reducer: [sliceA, sliceB] }) and let configureStore call combineSlices automatically.

It's worth noting that combineSlices supercedes combineReducers meaning that where configureStore currently calls combineReducers could be replaced with combineSlices. (this would also support doing configureStore({ reducer: fooSlice }) but I don't know if we want to support that types-wise)

let reducer: Reducer<S, A>;
if (typeof options.reducer === "object") {
  reducer = Array.isArray(options.reducer) ? combineSlices(...options.reducer) : combineSlices(options.reducer)
} else {
  reducer = options.reducer
}