Add .exe to commands on Windows

On windows, executable commands must have one of the extensions found in $env:PATHEXT. This branch creates a newCommand map where names contain the .exe extension.
I know this has already had several LGTMs, but I'm a bit concerned about the duplication of this map. Someone is bound to update one (most likely the Linux one) and not the other. I'd prefer if we just had a platform-specific suffix (="" for Linux) which we add to the map keys.