tor-config: Update to shellexpand 3.x

This is the new upstream version (published by me, recently). It has the same MSRV and one breaking change:

The caller who specifies a home dir function for substituting into strings, must now supply a string, not Path. Previously shellexpand would allow the caller to supply non-unicode data, and then simply not substitute it. That was an infelicity in the shellexpand API.

Now this infelicity is pushed into our code. The overall behaviour of Arti hasn't changed as a result. And it seems reasonable to me.

shellexpand 3.x also has a module for expanding Paths instead, in response to requests for this filed as upstream tickets. We could use that but I am not sanguine about that approach: the Pathness would spread throughout much of our config and file handling code.

I think we should at the very least postpone trying to work with invalid-unicode-paths as long as we can.