Scripting: batch add/remove layers

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**Is your feature request related to a problem? Please describe.**
I maintain a script that frequently adds/removes all layers on a map. At this time I have to call addLayer() and removeLayer() for each individual layer. For large maps this can be noticeably slow.

**Describe the solution you'd like**
I would like an addLayers(layers: Layer[]) function that takes an array of Layer[] to be added to the map.

I would also like a removeLayers() function that removes all layers, and a removeLayersAt(from: number, to: number) function that removes the layers between the provided indices.

**Describe alternatives you've considered**
No alternatives possible.

Start by locating the scripting entry points for addLayer() and removeLayer(). Review how individual layer operations are exposed, then determine the expected behavior for addLayers(layers), removeLayers(), and removeLayersAt(from, to), including index boundaries. Done means all three batch operations are available through scripting and match the requested layer behavior.

Written by the indexing model from the issue text.