refactor(example-app): remove store generic

[timdeschryver]

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[ ] Bugfix
[ ] Feature
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Other information

This PR removes the generic interface for Store.
Because of this change, I also had to change the createFeatureSelector method, otherwise it was giving compile erros that state doesn't match the interface.

[ValentinBossi]

before this commit with the signature export declare function createFeatureSelector<T, V>(featureName: keyof T): MemoizedSelector<T, V>; it was clear if the state is ok (having all the selected state keys) for selecting when composing two selectors from different createFeatureSelector. now if one feature key is lazy loaded and selected there will be an error, right? @timdeschryver
see also the discussion: #2849 (comment)