Add CI via GitHub Actions

[Croydon]

This adds some basic CI builds via GitHub Actions.

CMake

This produces a build matrix [Linux, Windows, MacOS] x [Debug, Release] and builds libmodplug with CMake with the default detected compiler in the respective GitHub Actions environment (currently: GCC 9, Visual Studio 2019 and Apple Clang 13).

Autotools

This produces a build matrix [Linux] x [Debug, Release] and build libmodplug with Autotools with the default detected compiler in the Ubuntu GitHub Actions environment (currently: GCC 9)

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An exemplary run can be seen here: https://github.com/Croydon/libmodplug/actions/runs/1772652809

Later on, this could be extended to cover more compilers, compiler versions and build systems, but it is a start to get some CI going.

If this is merged, GitHub will run this CI for every future push and pull request, no further configuration is required whatsoever.

[sezero]

AFAICS, cmake support here is still not fully mature (e.g. non-identical library output names for mingw and mac compared to autotools, maybe other issues as well), so, if it were me I'd use autotools for this.

[Croydon]

autotools could be added in a separate pull request as well, but I don't think that this is a reason against adding CMake coverage

[Croydon]

I have added Autotools builds