add optional layerNames property, so that you don't have to iterate over feature...

[todo]

https://github.com/kylebarron/deckgl-style-spec/blob/fdee88afbbf2defb0f7c58587cc8e86b6daa66a6/lib/parse-mapbox-style.js#L94-L98

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This issue was generated by todo based on a TODO comment in fdee88a. It's been assigned to @kylebarron because they committed the code.