create a nuget/winget installer

[aryairani]

On macOS we have a nice-ish Homebrew package for easy installation of Unison, and we basically understand how to do the same for Linux, but haven't gotten around to it ;-)

On Windows we could use some help — let us know if you're up to the challenge!

See: https://learn.microsoft.com/en-us/nuget/create-packages/creating-a-package