Extend object classes in TypeScript with generic types

[Berthur]

I propose these changes to make it possible to use generics for inherited three.js object types.

An example:

function getMesh(): Mesh<BufferGeometry> {
  /* Do stuff */
  return new InstancedMesh(geometry, material, n);
}

This will currently give the TS error Type 'InstancedMesh<BufferGeometry, Material>' is not assignable to type 'Mesh<BufferGeometry, Material | Material[]>'.

[mrdoob]

Thanks!