Add ISlashCommand interface for discord slash commands

src/main/java/me/goudham/bot/command/ISlashCommand.java

@@ -0,0 +1,7 @@
+package me.goudham.bot.command;
+
+import net.dv8tion.jda.api.events.interaction.SlashCommandEvent;
+
+public interface ISlashCommand {
+    void handle(SlashCommandEvent event);
+}