add $itemnotifs on proxy server

src/ReceiveSubcommands.hh

@@ -9,3 +9,10 @@
 
 void on_subcommand_multi(std::shared_ptr<Client> c, uint8_t command, uint8_t flag, std::string& data);
 bool subcommand_is_implemented(uint8_t which);
+
+void send_item_notification_if_needed(
+    std::shared_ptr<ServerState> s,
+    Channel& ch,
+    const Client::Config& config,
+    const ItemData& item,
+    bool is_from_rare_table);