TS:Improved cloneable class declaration

[Tonvey]

Hi,
I change the cloneable class typescript declaration.
In some cases , I need to write a class inherit from those cloneable class
and override the member function "clone" . It will raise a typescript compile error because
of the super class return type. In fact , the clone function return a new instance , not the instance "this"

[mrdoob]

Thanks!