Create YouTube "Radio" plugin

This should be a new plugin that is able to receive a YouTube URL from a user. The URL can either be to a single YouTube video or a video playlist. The plugin will receive the audio and play the audio on a voice channel.

The plugin will select which channel to play the audio by joining the voice channel the user is already connected to. This means the user must be connected to a voice channel prior to using the plugin.

Start by reading the repository's existing plugin structure and Discord voice-handling entry points. Determine how a plugin should accept a YouTube URL, handle both videos and playlists, join the user's voice channel, and play audio; done means these behaviors work for a user already connected to a voice channel.

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