Where a custom layer goes in the map's layer stack: a BeforeLayerConfig, or { layerID } to
go beneath a layer outside the map style, such as another layer of this module or one another
module added.
Remarks
A layerID no layer on the map has puts the layer on top, as MapLibre's addLayer does.
Where a custom layer goes in the map's layer stack: a BeforeLayerConfig, or
{ layerID }to go beneath a layer outside the map style, such as another layer of this module or one another module added.