Object3D: added attach() method

[WestLangley]

Object3D.attach( object ) adds object as a child of this, while maintaining the object's world transform.

Fixes #6426 and replaces #16380.

This method alleviates the need for THREE.SceneUtils.attach/detach(), which can be deprecated in a later PR.

newParent.attach( object ); // detaches from oldParent...

oldParent.attach( object ); // ...now it's back where it started

[mrdoob]

Thanks!