Add support for msys2 on windows

[dominicletz]

Thanks for the great plugin @kennyp ❤ ! This PR adds windows support when using the msys2 bash and linux tooling.

[kennyp]

@dominicletz mind updating .github/workflows/main.yml to include windows as well? Thanks!

[dominicletz]

Hm, would love to, but it looks like the asdf-vm/actions/plugin-test@v1 is not windows compatible at all. So someone will have to fix that, or replace the CI steps here with simple bash to test the plugin.