Add a plugin command example

src/main/java/xyz/grasscutters/pltm/commands/ExampleCommand.java

@@ -0,0 +1,43 @@
+package xyz.grasscutters.pltm.commands;
+
+import emu.grasscutter.command.Command;
+import emu.grasscutter.command.CommandHandler;
+import emu.grasscutter.game.player.Player;
+import xyz.grasscutters.pltm.PluginTemplate;
+
+import java.util.List;
+
+/**
+ * Commands are comprised of 3 things:
+ * 1. The {@link Command} annotation.
+ * 2. Implementing the {@link CommandHandler} interface.
+ * 3. Implementing the {@link CommandHandler#execute(Player, Player, List)} method.
+ * 
+ * The {@link Command} annotation should contain:
+ * 1. A command label. ('example' in this case makes '/example' runnable in-game or on the console)
+ * 2. A description of the command. (this is shown in the `/help` command description)
+ * 3. A permission node. (this is used to check if the player has permission to use the command)
+ * Other optional fields can be found in the {@link Command} annotation interface.
+ */
+
+@Command(label = "example", description = "An example of a plugin command.", 
+        usage = "example <toLog>", permission = "pltm.example")
+public final class ExampleCommand implements CommandHandler {
+    /**
+     * Called when `/example` is run either in-game or on the server console.
+     * @param sender The player/console that invoked the command.
+     * @param targetPlayer The player that the sender was targeting.
+     * @param args The arguments to the command.
+     *             
+     * If {@param sender} is null, the command was run on the server console.
+     * {@param targetPlayer} can be null if the sender did not specify a target.
+     * For sending feedback to the sender, use {@link CommandHandler#sendMessage(Player, String)}
+     */
+    @Override public void execute(Player sender, Player targetPlayer, List<String> args) {
+        var joined = String.join(" ", args);
+        PluginTemplate.getInstance().getLogger().info(joined);
+        
+        if(sender != null)
+            CommandHandler.sendMessage(sender, "Logged the message to the console.");
+    }
+}