Add C++ FFI example

Both `extern` functions to show normal C API usage in C++, but also binding to C++ functions with name mangling and C++ APIs, like returning class instances that have nontrivial copy constructors.

Look at existing FFI examples in the denonomicon repository to understand the structure. The task involves creating examples for both calling C functions from TypeScript/Deno and calling C++ functions (handling name mangling and complex types). Start by examining the Deno FFI documentation and the repository's example directory. 'Done' means adding clear, runnable code examples that demonstrate these concepts.

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