Add alternative variable reference syntax for NSExpression

[1ec5]

The NSExpression-based property value implementation in #10726 uses the $dollar syntax to represent both references to variables and the built-in variables $zoomLevel and $heatmapDensity. (Natively, $dollar syntax is used for references to state that may change from one evaluation context to another.) In the unlikely event that a style needs to refer to a custom zoomLevel or heatmapDensity variable, we should have a fallback syntax, such as a custom variable function.

[1ec5]

As of #11472, MGL_FUNCTION('var', 'zoom') is available in case the developer needs to refer to a variable named zoomLevel rather than the actual zoom level.