add
@NonNull()
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Adds a new [style custom layer](https://docs.mapbox.com/mapbox-gl-js/style-spec/#layers). Runtime style layers are valid until they are either removed or a new style is loaded.
Return
string describing an error if the operation was not successful, or empty otherwise.
Parameters
layerHost
The `custom layer host`.
layerId
A style layer identifier.
layerPosition
If not empty, the new layer will be positioned according to `layer position` parameters.