Extract the `initialState` property out of the `config` object passed
to `createStoreModule` into a separate argument which is passed before
`config`. This allows TypeScript to infer the type of fields/parameters
etc. which refer to the module state while processing the `config`
argument.

For example, given the following:

```
export default createStoreModule(initialState, {
  reducers: {
    updateThing(state, action) {
      ...
    }
  }
});
```

TS can infer the type of the `state` argument to `updateThing` without
needing to explicitly annotate it. The same approach can work for other
fields as well. This inference does not work when `initialState` is a
property of the `config` object however.