Finish command completion listener implementation

Very similarly to how the slash command interface works, now a slash command auto-completion interface also exists, with its respective listener.

src/main/java/wtf/beatrice/hidekobot/objects/commands/SlashArgumentsCompleter.java

@@ -0,0 +1,21 @@
+package wtf.beatrice.hidekobot.objects.commands;
+
+import net.dv8tion.jda.api.events.interaction.command.CommandAutoCompleteInteractionEvent;
+import org.jetbrains.annotations.NotNull;
+
+public interface SlashArgumentsCompleter
+{
+
+    /**
+     * Get the parent slash command's object.
+     *
+     * @return the command object.
+     */
+    SlashCommand getCommand();
+    /**
+     * Run the argument-completion logic by parsing the event and replying accordingly.
+     *
+     * @param event the received auto-complete event.
+     */
+    void runCompletion(@NotNull CommandAutoCompleteInteractionEvent event);
+}

src/main/java/wtf/beatrice/hidekobot/objects/commands/SlashArgumentsCompleterImpl.java

@@ -0,0 +1,21 @@
+package wtf.beatrice.hidekobot.objects.commands;
+
+import net.dv8tion.jda.api.events.interaction.command.CommandAutoCompleteInteractionEvent;
+import org.jetbrains.annotations.NotNull;
+
+public class SlashArgumentsCompleterImpl implements SlashArgumentsCompleter
+{
+    private final SlashCommand parentCommand;
+    public SlashArgumentsCompleterImpl(SlashCommand parentCommand)
+    {
+        this.parentCommand = parentCommand;
+    }
+
+    public SlashCommand getCommand()
+    { return parentCommand; }
+
+    public void runCompletion(@NotNull CommandAutoCompleteInteractionEvent event)
+    {
+        return;
+    }
+}