WIP: Migrate pcbuild.sln to pcbuild.slnx

[edvilme]

VS 17.13+ introduces a new .slnx file format for solutions that is easier to read and parse, and is less propense to merge conflicts. This pr proposes a migration to the new format.

[zooba]

At some point we have to migrate from the PCbuild directory to Platforms\Windows, and so that's probably the right time to change a filename that people may be relying on. I'd prefer to combine the breaking changes.

Of course, that whole migration is probably going to be a slow painful process, but this particular file should be able to be one of the first to safely move. We can also rename it from "pcbuild" at that point, presumably to CPython.slnx.

So yes, we will likely do this, but not on its own. Thanks for making a start!