Add initial support for wasm32-emscripten target

This patchset adds very basic support for vlc compilation into the wasm32-emscripten target, it will allow adding contribs and modules incrementally with a CI ensuring no breaking changes are added.

It will add stubs for getProxyUrl, implement a function to get the thread_id and use the macos implementation of clock_nanosleep to circumvent the fact that it is missing in emscripten.

It also adds support for dav1d, ffmpeg, openjpeg and libvpx contribs.