Add list command

Signed-off-by: Jaida Wu <mlgmxyysd@meowcat.org>

src/main/java/emu/grasscutter/command/commands/ListCommand.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 java.util.List;
+import java.util.Map;
+
+@Command(label = "list", description = "List online players")
+public class ListCommand implements CommandHandler {
+
+    @Override
+    public void execute(GenshinPlayer sender, List<String> args) {
+        Map<Integer, GenshinPlayer> playersMap = Grasscutter.getGameServer().getPlayers();
+
+        CommandHandler.sendMessage(sender, String.format("There are %s player(s) online:", playersMap.size()));
+
+        if (playersMap.size() != 0) {
+            StringBuilder playerSet = new StringBuilder();
+
+            for (Map.Entry<Integer, GenshinPlayer> entry : playersMap.entrySet()) {
+                playerSet.append(entry.getValue().getNickname());
+                playerSet.append(", ");
+            }
+
+            String players = playerSet.toString();
+
+            CommandHandler.sendMessage(sender, players.substring(0, players.length() - 2));
+        }
+    }
+}