Add GitHub Actions CI

[Croydon]

This adds some basic CI builds via GitHub Actions.

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

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

An exemplary run can be seen here: https://github.com/Croydon/enet/actions/runs/1723356026

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