Add windows instance flag support

In #6789 we added posix support for the instance flag.

It was determined Windows support required more design/discussion.

The biggest design issue is around environment variable naming and templating.

In windows, the flag for the socket is -namedPipe. Do we want the template variable names to have socket in the name, or something like named pipe?

The second question is around the variable format used in the template. we use %i in the posix version, which would work well on posix compatible shells on windows, but may run into problems with cmd? I don't know enough about it to be sure.