Make Mod an abstract class (#5155)

core/src/mindustry/mod/Mod.java

@@ -4,7 +4,7 @@ import arc.files.*;
 import arc.util.*;
 import mindustry.*;
 
-public class Mod{
+public abstract class Mod{
     /** @return the config file for this plugin, as the file 'mods/[plugin-name]/config.json'.*/
     public Fi getConfig(){
         return Vars.mods.getConfig(this);