OS::create() should use template in dynamic cast

As seen here:

https://github.com/unitpoint/objectscript/blob/master/src/objectscript.h#L3098-L3103

the `dynamic_cast` refers to the actual `OS` class itself, making subclassing a bit harder, as one has to implement this method themself. I had to do this in my case.

If possible, the template `T` should be used for the cast, so that derived classes may not need to re-implement the creation. :)
