Windows support

Motivation

Users have frequently requested windows support of ghcup. Recent discussions have shown that the Haskell Foundation is pushing for installers that support all 3 platforms (linux, mac, windows) consistently.

We believe that ghcup with its unix philosophy model and built around the principles of rustup is the right approach for fixing these concerns.

Implementation plan

1. make the codebase build on windows (see WIP PR)
2. manage MSys2
3. create an installer like rustup (powershell wrapper)

Specification

Interface

Windows will have the same interface as unix, including command line options and directory structure.

MSys2

TODO

Installer

TODO