Set ItemManager index to correct value after ReceivedItems (#193)

I noticed in the CrossCode APWorld that every time an item was received,
my client would send a `Sync` Packet and the server would, naturally,
respond with a `ReceivedItems` packet with `index=0`. This turned out to
be an oversight in `ItemsManager.#onReceivedItems` in which the program
would fail to update `#index` to the new value, therefore keeping
`#index` constantly at zero, and creating some unnecessary network
bandwidth. This PR adds a whopping one line of code that should fix this
issue.

src/managers/ItemsManager.ts

@@ -128,5 +128,7 @@ export class ItemsManager {
         for (const item of packet.items) {
             this.#items[index++] = item;
         }
+
+        this.#index = index;
     }
 }