[Experimental] Unity native prefab importer

! this is not fully functional so far, just a milestone

+ imports scml file as a prefab, similar to the one created by python
version
+ all you have to do is to put content of Experimental directory in
your Asset Unity folder
+ importing / reimporting SCML files creates S2UOutput directory under
assets with imported prefab
+ wasn’t deeply tested yet and can change wildly