3.0.0

This release upgrades MapboxGL.js to 0.50.0. It also fixes some issues where the types had some limitations.

Full diff:

---- Mapbox.Expression - MINOR ----

    Added:
        type SymbolZOrder 
        orderSource :
            Expression exprType SymbolZOrder
        orderViewportY :
            Expression exprType SymbolZOrder


---- Mapbox.Layer - MAJOR ----

    Added:
        fillExtrusionVerticalGradient :
            Expression
                CameraExpression
                Basics.Bool
            -> LayerAttr FillExtrusion
        symbolZOrder :
            Expression
                CameraExpression
                SymbolZOrder
            -> LayerAttr Symbol
    
    Changed:
      - backgroundPattern : Expression any String -> LayerAttr Background
      + backgroundPattern : Expression CameraExpression String -> LayerAttr Background
    


---- Mapbox.Source - MAJOR ----

    Changed:
      - scheme : Scheme -> SourceOption RasterSource
      + scheme : Scheme -> SourceOption any