Implement TIP #440

[dbohdan]

Hey there! Could you implement tcl_platform(engine) from TIP #440 to make it possible for Tcl code to tell if it's running in ParTcl?

P.S.: It's always cool to see another tiny Tcl implementation. I've add yours to the list at https://tcl.wiki/Small%20Tcl.

[dbohdan]

One thing to note is that if you set tcl_platform(engine) to ParTcl your implementation may be confused with http://github.com/partcl/partcl. This may not be a problem, however, considering that the latter hasn't seen any development since 2012 and may never get TIP #440 support.