feat: dispatch layerchange event

[silvester-pari]

Implemented changes

This PR adds a generic layerchange event to eox-layercontrol. Currently it triggers when a layer's visibility is changed or the length of layers inside a group is changed; more events could be added in the future.

Usage:

eoxLayerControl.addEventListener("layerchange", (e) => {
  console.log(e.detail.get("id))  
  console.log(e.detail.getVisible())
})

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• I have performed a self-review of my code
• I have added a test related to this feature/fix
• I have added documentation
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[viktoraronfarkas]

looks good, thanks! :)