Add killcharacter command

src/main/java/emu/grasscutter/command/commands/KillCharacterCommand.java

+package emu.grasscutter.command.commands;
+
+import emu.grasscutter.Grasscutter;
+import emu.grasscutter.command.Command;
+import emu.grasscutter.command.CommandHandler;
+import emu.grasscutter.game.GenshinPlayer;
+import emu.grasscutter.game.entity.EntityAvatar;
+import emu.grasscutter.game.props.FightProperty;
+import emu.grasscutter.game.props.LifeState;
+import emu.grasscutter.server.packet.send.PacketEntityFightPropUpdateNotify;
+import emu.grasscutter.server.packet.send.PacketLifeStateChangeNotify;
+
+import java.util.List;
+
+@Command(label = "killcharacter", usage = "killcharacter [playerId]", aliases = {"suicide"},
+        description = "Directly kill a player's current active character", permission = "player.killcharacter")
+public final class KillCharacterCommand implements CommandHandler {
+
+    @Override
+    public void execute(GenshinPlayer sender, List<String> args) {
+        if (sender == null) {
+            CommandHandler.sendMessage(null, "Run this command in-game.");
+            return; // TODO: kill a player's current active character from console
+        }
+
+        int target;
+        if (args.size() == 1) {
+            try {
+                target = Integer.parseInt(args.get(0));
+                if (Grasscutter.getGameServer().getPlayerByUid(target) == null) {
+                    target = sender.getUid();
+                }
+            } catch (NumberFormatException e) {
+                CommandHandler.sendMessage(sender, "Invalid player id.");
+                return;
+            }
+        } else {
+            target = sender.getUid();
+        }
+        GenshinPlayer targetPlayer = Grasscutter.getGameServer().getPlayerByUid(target);
+        if (targetPlayer == null) {
+            CommandHandler.sendMessage(sender, "Player not found or offline.");
+            return;
+        }
+
+        EntityAvatar entity = targetPlayer.getTeamManager().getCurrentAvatarEntity();
+        entity.setFightProperty(FightProperty.FIGHT_PROP_CUR_HP, 0f);
+        // Packets
+        entity.getWorld().broadcastPacket(new PacketEntityFightPropUpdateNotify(entity, FightProperty.FIGHT_PROP_CUR_HP));
+        entity.getWorld().broadcastPacket(new PacketLifeStateChangeNotify(0, entity, LifeState.LIFE_DEAD));
+        // remove
+        targetPlayer.getScene().removeEntity(entity);
+        entity.onDeath(0);
+
+        sender.dropMessage("Killed " + targetPlayer.getNickname() + "'s current active character.");
+    }
+}