Limited POSIX threading wrapper for FreeRTOS and a working Visual Studio demo project.

Please be aware right after tagging this release, the repo was re-structure. Demo project is now moved to https://github.com/FreeRTOS/FreeRTOS-Labs.

Lab code and projects are undergoing optimizations or refactorization to improve memory usage, modularity, documentation, demo usability, or test coverage. Please expect more changes including directory restructure in this repo.