Refactor and simplify the main bot classes

PluralKit.Bot/Handlers/IEventHandler.cs

@@ -0,0 +1,14 @@
+using System.Threading.Tasks;
+
+using DSharpPlus.Entities;
+using DSharpPlus.EventArgs;
+
+namespace PluralKit.Bot
+{
+    public interface IEventHandler<in T> where T: DiscordEventArgs
+    {
+        Task Handle(T evt);
+
+        DiscordChannel ErrorChannelFor(T evt) => null;
+    }
+}