add Windows build support

[daschiller]

Tested and working.

I did not add it to the default target or to the CI, since I am not sure if you want to release Windows binaries.

[mostlygeek]

Windows isn't well supported by llama-swap because it expects Unix style processes and signals. The project would need some windows specific swapping tests before distributing a build for it.

[daschiller]

I've been running it for the past few days without any issues. It can swap between models and kill spawned processes after TTL expiry. The only problem I ran into was having to escape backslashes for paths in the cmd option, but that's not unusual.

[mostlygeek]

I managed to test it on a windows PC today and it worked better than I expected it to. Much to my surprise. Thanks for the PR.

Also adding a windows binary to the release build since it seems to work well enough.