Handling C++: C _and_ Dart bindings generator

For dealing with C++ code bases we could consider generating both C and Dart files.

Of course those generated C files would need to be hooked back in to the C build, but it would prevent users having to write C wrappers manually for C++ types such as Vectors.

I'm not sure if we should do this in the scope of package:ffigen, or whether this should be another package (which could reuse the ffigen clang bindings).

@mannprerak2, based on your experience, would it be beneficial to have it inside this package for code reuse?

This would probably be a larger project that requires thinking about how to deal with the different data structures.

Context: https://github.com/dart-lang/ffigen/issues/208, https://github.com/dart-lang/sdk/issues/38788