Provide an installer and an install target on Windows

[kit-ty-kate]

Noted in https://discuss.ocaml.org/t/ann-opam-2-2-0-beta1/13913/2

Currently make {cold-,}install only installs things to the Cygwin directories, however we probably want for it to be installed outside when the binary is not a cygwin binary (mingw, msvc)

On top of that we should probably provide a typical Windows installer (or at least a cmd/powershell script if we want to go the install.sh route), although last time i’ve done that was 15 years ago so I’m a bit rusty.

[kit-ty-kate]

Note for dev meeting: we think a powershell script would be good as a windows equivalent of shell/install.sh