Rename $featureProperties to $featureAttributes

[1ec5]

The $featureProperties expression variable should be renamed $featureAttributes to match the MGLFeature.attributes property. The iOS/macOS map SDK avoids using the term “property” to refer to things that aren’t represented by class or instance properties in Objective-C or Swift.

If this change doesn’t take place before the iOS map SDK v4.0.0 release, we can support both variable names but prefer $featureAttributes. ($featureProperties isn’t documented, in any case.) The NSExpression.featurePropertiesVariableExpression property would ideally be renamed as well; we’d have to leave a deprecated alias in place for it.

/cc @fabian-guerra

[fabian-guerra]

Fixed in #11748