Third-party library repositories on Windows CI

[HertzDevil]

Our Windows CI workflow tries to use the original library repositories to build Crystal's dependencies whenever possible, but we are still left with a few third-party repos in use:

• https://github.com/winlibs/libffi
• https://github.com/pffang/libiconv-for-Windows.git
• https://github.com/BrianGladman/mpir.git (MPIR itself is an unmaintained fork of GMP)

These all add MSBuild support since their upstreams do not support the MSVC toolchain out of the box. However, they might become unmaintained, tempered, or even deleted, potentially compromising our Windows builds. So I think we need to act:

• Clone the upstreams in crystal-lang and maintain the MSVC projects ourselves directly;
• Figure out whether building with MSYS(2) on CI is possible;
• Somehow get the upstreams to provide MSVC toolchain support.

[ysbaddaden]

Figure out whether building with MSYS(2) on CI is possible;

You mean installing the libraries as dependencies from MSYS(2) repos (mingw/ucrt?) to retain compatibility with MSVC? Or to build the libraries in a MSYS(2) environment?

[HertzDevil]

The latter, as we have always done