Add CMake as an alternative build system alongside the existing qmake.

CMake is currently the de facto standard for C++ projects with >50% market share (from JetBrains annual survey). I don't plan to switch away from qmake for production anytime soon. But CMake does offers several advantages:

• Better IDE integration. Native support from Visual Studio, CLion, VSCode, and even QtCreator
• Easier integration with package managers like vcpkg and Conan
• Better tool integration, e.g., clang-tidy, sanitisers, include-what-you-use
• CPack, a potentially easier approach to creating Windows installers

It won't cost too much to maintain both build systems after the initial setup.

Qt made a tool qmake2cmake to help with this.