Merge vlcrs-plugin into vlcrs-core
vlcrs_plugin exposes the base types to create plugin, much like vlc_plugin.h in the C headers, and uses the vlcrs_core::object::Object type. But vlcrs_core also needs vlcrs_plugin internally to define the capabilities, and both need each other for tests also.
The same applies to vlcrs_macros when used in tests, which must stay separated, but we can still provide it as a dev-dependencies and avoid the cyclic reference.