Add abstract Mapbox Layer

[dellisd]

Create the base of a class hierarchy for representing Layers in Mapbox. Attributes that are common to all types of layers include an id, source, filter (an expression), layout (expressions), paint (expressions), minzoom and maxzoom, and others listed in the Mapbox docs.

This should be an abstract, expected class in the mapbox module's commonMain sourceset.

Provide actual implementations for Android and JS, as with the GeoJsonSource.