SPIGOT-6829: Add per-player world border API

By: Parker Hawke <hawkeboyz2@hotmail.com>

paper-server/src/main/java/org/bukkit/craftbukkit/CraftServer.java

@@ -127,6 +127,7 @@ import org.bukkit.UnsafeValues;
 import org.bukkit.Warning.WarningState;
 import org.bukkit.World;
 import org.bukkit.World.Environment;
+import org.bukkit.WorldBorder;
 import org.bukkit.WorldCreator;
 import org.bukkit.block.data.BlockData;
 import org.bukkit.boss.BarColor;
@@ -1184,6 +1185,11 @@ public final class CraftServer implements Server {
         worlds.put(world.getName().toLowerCase(java.util.Locale.ENGLISH), world);
     }
 
+    @Override
+    public WorldBorder createWorldBorder() {
+        return new CraftWorldBorder(new net.minecraft.world.level.border.WorldBorder());
+    }
+
     @Override
     public Logger getLogger() {
         return logger;