userStore should be StateStore instead of WebStorageStateStore allowing custom implementation

[sebastienlabine]

Hi.

I'm trying to customize the stateStore for my react native implementation, but typescript expects a WebStorageStateStore which doesn't work.

However, the stateStore is configured properly.

stateStore: StateStore;
userStore: WebStorageStateStore;

We would have

stateStore: StateStore;
userStore: StateStore;