Add Missing Types to the Store Walkthrough Section

[markostanimirovic]

Add missing types to the Store Walkthrough section:

Change underlined code to:

@Output() add = new EventEmitter<string>();

Change underlined code to:

@Output remove = new EventEmitter<string>();

Change underlined code to:

onAdd(bookId: string): void {
  // ...
}

onRemove(bookId: string): void {
  // ...
}

[muhammadawaisshaikh]

assign it to me please.

[muhammadawaisshaikh]

hey @markostanimirovic, please review and merge (#3206), thanks