Feat: splitChannels overlay

[katspaugh]

Short description

Resolves #3565

Implementation details

Adds an overlay option for splitChannels.

Example usage:

const wavesurfer = WaveSurfer.create({
  container: document.body,
  url: '/examples/audio/stereo.mp3',
  splitChannels: [
    {
      waveColor: 'rgb(200, 0, 200)',
      progressColor: 'rgb(100, 0, 100)',
    },
    {
      waveColor: 'rgb(0, 200, 200)',
      progressColor: 'rgb(0, 100, 100)',
      overlay: true
    },
  ],
})

☝️ the second channel will be overlayed on top of the first one.

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