[[Bugfix 21530]] Add more FFI examples to the Extending LiveCode Guide

[andrewferguson]

I've updated the C FFI section of the "Extending LiveCode" guide to include a couple more examples of wrapping C functions (including an example of wrapping one of Apple's C-based APIs) and a guide to utilizing the already-written C/C++ functions that are available through LiveCode's foundation library (libfoundation).

[livecodepanos]

@livecodeali do you think this can be rebased against develop-9.0? It would be good to have it in LC 9.0.2 rc-1 too

[livecodeali]

@livecodepanos Yes, I don't see why not