Support emscripten

Similar to llvm, install emscripten in ~/emscripten (?)

• Install on Linux
  • Ubuntu: apt-get install emscripten ?
  • Arch: https://archlinux.org/packages/community/x86_64/emscripten/
  • Fedora: dnf install emscripten ?
• Windows: https://community.chocolatey.org/packages/emscripten ???
• MacOS: https://formulae.brew.sh/formula/emscripten ???
• add ./emsdk_env.sh in to ~/.cpprc (?) @aminya

See docker in project_options ...

# Get the emsdk repo
git clone https://github.com/emscripten-core/emsdk.git /root/emsdk
# Download and install the latest SDK tools.
/root/emsdk/emsdk install latest
# Make the "latest" SDK "active" for the current user. (writes .emscripten file)
/root/emsdk/emsdk activate latest
# Get emscripten repo, for Emscripten.cmake
git clone https://github.com/emscripten-core/emscripten /root/emscripten

Example:

.\setup_cpp_windows --compiler emscripten --cmake true --ninja true --ccache true --vcpkg true

Start by comparing the existing LLVM setup with the setup_cpp_windows --compiler emscripten example and the linked docker/Dockerfile.emscripten. Resolve installation and environment setup for Linux, Windows, and macOS, including the proposed ~/.cpprc entry. Done means the Emscripten compiler setup works across the listed platforms with the documented CMake, Ninja, ccache, and vcpkg options.