Move icons to gamemode and add icons for vanilla modes

MiraAPI.Example/GameModes/ExampleMode.cs

@@ -1,4 +1,5 @@
 using MiraAPI.GameModes;
+using MiraAPI.Utilities.Assets;
 using UnityEngine;
 
 namespace MiraAPI.Example.GameModes;
@@ -7,5 +8,6 @@ public class ExampleMode : AbstractGameMode
 {
     public override string Name => "Example Mode";
     public override string Description => "An example gamemode.";
+    public override LoadableAsset<Sprite>? Icon => ExampleAssets.ExampleButton;
     public override Color Color => Color.red;
 }