Docs: Store/State class pattern

[azu]

We often write following Store and State class pattern.

It should be documentation as implementation example.

class MyState {
   update(){}
}

class MyStore extends Store<MyState>{
  constructor(){
    super();
    this.state = new MyState();
  }
  receivePayload(){
    this.setState(this.state.update())
  }
  getState(){
    return this.state;
  }
}

Examples:

• https://github.com/almin/almin/tree/master/examples/shopping-cart/src/stores

TypeScript enhance this pattern:

• Strict property initialization checks in classes by ahejlsberg · Pull Request #20075 · Microsoft/TypeScript