Feat: [Regions] emit region-update event

[katspaugh]

Short description

Resolves #3779

Implementation details

A new Regions event is added:

  /** When a region is being updated */
  'region-update': [region: Region, side?: 'start' | 'end']

Example usage

import WaveSurfer from 'wavesurfer.js'
import RegionsPlugin from 'wavesurfer.js/dist/plugins/regions.esm.js'

// Initialize the Regions plugin
const regions = RegionsPlugin.create()

// Create an instance of WaveSurfer
const wavesurfer = WaveSurfer.create({
  container: '#waveform',
  url: '/examples/audio/audio.wav',
  plugins: [regions],
})

regions.on('region-update', (region, side) => {
  console.log('A region is being dragged or resized', region)
})

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