Made the onLoad param a callback generic

Turned the onLoad parameter of the ObjectLoader.load function a generic type of Object3D so users can define which kind of object they are loaded so it stays typed.

src/loaders/ObjectLoader.d.ts

@@ -13,7 +13,7 @@ export class ObjectLoader {
 
   load(
     url: string,
-    onLoad?: (object: Object3D) => void,
+    onLoad?: <ObjectType extends Object3D>(object: ObjectType) => void,
     onProgress?: (event: ProgressEvent) => void,
     onError?: (event: Error | ErrorEvent) => void
   ): void;