MinGW-w64 / MSYS2 support

[HertzDevil]

As part of crystal-lang/crystal#6170, it makes sense to test Shards on MSYS2 and package Shards for it as well. The bad news is that all the resolvers are problematic on MSYS2:

• Mercurial is only available from the MSYS environment, and installs /usr/bin/hg, a Python script with a shebang, which Process.run doesn't handle. Nor is running the script with Python expected on Windows, since Mercurial packages hg.exe using PyOxidizer; the specs will pass if this executable is exposed inside MSYS2 with something like PATH="$(cygpath $PROGRAMFILES)/Mercurial:$PATH".
• Fossil has a UCRT64 build but it is completely broken. It is a single executable, so downloading the official Windows build somewhere to $PATH would work. (If obtained via WinGet, it should be available at $(cygpath $LOCALAPPDATA)/Microsoft/WinGet/Links.)
• No specs are affected by MSYS Git, but it has its own issues.

Everything else seems to work fine.

[straight-shoota]

I understand these issues are predominantly for setting up CI testing on MSYS2? It's not so much a problem for using it?

[HertzDevil]

It affects the check step within a PKGBUILD file too, since tests running on MSYS2's own CI naturally depend only on other MSYS2 packages. Basically we have to add skip_fossil=1 skip_hg=1 to make test; perhaps we could add a special case for hg when $MSYSTEM is defined, but I don't know if that is worth the effort (even as a patch exclusive to the MSYS2 package repository).

[ysbaddaden]

I assume the same issues will arise for normal usages too.