Make InventoryThingParser package-private.

We don't need the parser exposed outside of the `things` package, and
none of the other thing parsers are public anyway. Coincidentally, this
fixes a warning about exposing InventoryThing outside of its visibility
scope, so yay.

src/main/java/com/garbagemule/MobArena/things/InventoryThingParser.java

@@ -6,7 +6,7 @@ import org.bukkit.World;
 
 import java.util.function.Supplier;
 
-public class InventoryThingParser implements ThingParser {
+class InventoryThingParser implements ThingParser {
 
     private static final String PREFIX = "inv(";
     private static final String SUFFIX = ")";