Add: Indonesia Rupiah as in-game currency (#8616)

The exchange rate is actually more like 19,000, but OpenTTD
doesn't handle such high exchange rates kindly.

src/currency.h

@@ -63,6 +63,7 @@ enum Currencies {
 	CURRENCY_CNY,       ///< Chinese Renminbi
 	CURRENCY_HKD,       ///< Hong Kong Dollar
 	CURRENCY_INR,       ///< Indian Rupee
+	CURRENCY_IDR,       ///< Indonesian Rupiah
 	CURRENCY_END,       ///< always the last item
 };