Add no-op speech service

[Earthcomputer]

Description of proposed feature

Add a speech service which creates no sound at all. This could be useful to disable the voiceover and prevent it from getting in the way while you're only working on the video.

How can the new feature be used?

class MyVoiceoverScene(VoiceoverScene):
   def construct(self):
      # use no-op services for now while only working on visuals
      self.set_speech_service(NoOpService())
      #self.set_speech_service(GTTSService())
      #self.set_speech_service(RecorderService())

Additional comments

Maybe the no-op service could estimate the length of the audio using the length of the string as a heuristic, so wait_for_voiceover still works as expected.