Add NewGRF feature: Extra station name strings

These are used when all the default names have been used up,
instead of "Town Station #NNN".

src/saveload/extended_ver_sl.cpp

@@ -153,6 +153,7 @@ const SlxiSubChunkInfo _sl_xv_sub_chunk_infos[] = {
 	{ XSLFI_CUSTOM_TOWN_ZONE,       XSCF_IGNORABLE_UNKNOWN,   1,   1, "custom_town_zone",          nullptr, nullptr, nullptr        },
 	{ XSLFI_STATION_CARGO_HISTORY,  XSCF_NULL,                1,   1, "station_cargo_history",     nullptr, nullptr, nullptr        },
 	{ XSLFI_TRAIN_SPEED_ADAPTATION, XSCF_NULL,                1,   1, "train_speed_adaptation",    nullptr, nullptr, "TSAS"         },
+	{ XSLFI_EXTRA_STATION_NAMES,    XSCF_NULL,                1,   1, "extra_station_names",       nullptr, nullptr, nullptr        },
 	{ XSLFI_NULL, XSCF_NULL, 0, 0, nullptr, nullptr, nullptr, nullptr },// This is the end marker
 };