Upgrade node-mpv
Currently we're using a fork I made but the author recently updated its own branch and released a beta version to npm which we could use.
It includes the fixes I had made in my fork.
It has a few changes we need to be careful of though : https://github.com/j-holub/Node-MPV/blob/Node-MPV-2/migrationguide.md
Most of these changes are already used by us but there has been a few property and method changes we need to be careful of.