This pull request introduces a comprehensive C++ build and test system for the Arcscript transpiler, adding CMake-based build configuration, CI workflow, and improved project documentation. The changes make the project easier to build, test, and maintain across platforms, particularly Windows and macOS, and ensure proper handling of ANTLR4 dependencies.

Build System and CI Integration:

• Added a complete CMakeLists.txt for the C++ transpiler, supporting shared library and test executable builds, cross-platform output directories, external ANTLR4 integration, and automated copying of dependencies and headers.
• Introduced GitHub Actions workflow .github/workflows/cpp-build-test.yml to automatically build and test the C++ transpiler on Windows and macOS for pushes and pull requests.

Dependency Management:

• Added cmake/ExternalAntlr4Cpp.cmake for automated fetching and building of the ANTLR4 C++ runtime as an external project, with platform-specific handling.
• Added cmake/FindANTLR.cmake to locate the ANTLR jar and Java runtime, and provide a macro for generating C++ parser sources from grammar files within CMake.

Project Documentation and Housekeeping:

• Created a README.md in the Cpp directory with build instructions and prerequisites for Windows and Linux.
• Updated .gitignore to exclude IDE files, build artifacts, and CMake-generated directories.