Fix AreaStore's IDs persistence (#8888)

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Fix free ID function. Return first gap in map

builtin/game/features.lua

@@ -14,6 +14,7 @@ core.features = {
 	object_independent_selectionbox = true,
 	httpfetch_binary_data = true,
 	formspec_version_element = true,
+	area_store_persistent_ids = true,
 }
 
 function core.has_feature(arg)