Draft: [RFC]: lib: Add a libvlc_has_module helper

The purpose of this MR is to allow a libvlc application to know if a feature (or rather a module in the current implementation) is available.

My main motivation here is that the media library is starting to handle various online sources, such as podcasts, and is likely to be extended to other kind of services.

In order to expose allow the application to know what's supported, we need to probe the available modules. For instance, there's no point in exposing a youtube subscription if neither lua nor youtube-dl are enabled.