add registerclasses virtual method (closes #5)

Author: devpikachu

CHANGELOG.md

@@ -1,7 +1,12 @@
 ## 0.6.0
+**:boom: Breaking Changes**
+- Made `System.ShouldLoad` abstract ([#6](https://git.omni.ms/vintage-story-mods/common-mod/issues/6))
+
+**:sparkles: Features**
+- Added `System.RegisterClasses` virtual method ([#5](https://git.omni.ms/vintage-story-mods/common-mod/issues/5))
+
 **:hammer: Refactors**
 - Cleanup pass ([#1](https://git.omni.ms/vintage-story-mods/common-mod/issues/1))
-- Made `System.ShouldLoad` abstract ([#6](https://git.omni.ms/vintage-story-mods/common-mod/issues/6))
 
 **:books: Documentation**
 - Created skeleton documentation

Common.Mod/System.cs

@@ -141,6 +141,11 @@ public override void AssetsFinalize(ICoreAPI api)
         OnAssetsFinalized?.Invoke();
     }
 
+    public override void Start(ICoreAPI api)
+    {
+        RegisterClasses(api);
+    }
+
     public override void StartServerSide(ICoreServerAPI api)
     {
         api.Event.PlayerJoin += _OnServerPlayerJoined;
@@ -189,6 +194,11 @@ protected virtual void ClientStartPre(ICoreClientAPI api)
     {
     }
 
+    [UsedImplicitly]
+    protected virtual void RegisterClasses(ICoreAPI api)
+    {
+    }
+
     private void _OnServerPlayerJoined(IServerPlayer player)
     {
         // Config synchronization