CMake files to build with Emscripten targeting the web

[core-explorer]

1. install emscripten (I get it from https://github.com/emscripten-core/emsdk)
2. create a build directory
3. run cmake in the build directory for a normal build, it needs as an argument the directory containg CMakeLists.txt
4. run cmake in the build directory with additional argument -DCMAKE_TOOLCHAIN_FILE=cmake/Toolchain-wasm32-web.cmake to build with emscripten output should be index.html
5. serve index.html (I use python -m http.server)
6. parse C++ declarations in the browser
7. the CMake file represents minimal effort and not a full conversion of the meson file