add Style Layer
@NonNull()
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Adds a new [style 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
layerPosition
If not empty, the new layer will be positioned according to `layer position` parameters.
properties
A map of style layer properties.