Adding an action and preset to leave the currently active Voicechannel

[JMLutra]

Resolving one part of my feature request #17 I sadly haven't worked with ts ever before so I hope everything checks out.

[thedist]

Looks good to me, the only thing I'm going to add is that the interface for the action should have options: Record<string, never>, as even if the action doesn't require any options there'll still be an options object on callback that'll just be empty.