More robust support for feature.id

[bcamper]

All data formats supported by Tangram have a feature.id property (at the top feature level, outside feature.properties; some data sets may separately include an id or similar property within feature.properties), but we haven't had full support for it. This PR fixes that:

• Ensures feature.id persists wherever features are internally copied/synthesized/etc. internally
• Provides access to feature.id in user-authored JS scene functions with a new $id variable
• Includes feature.id in feature object results returned from scene.queryFeatures()
• Includes feature.id in the default uniqueness check for scene.queryFeatures(), with $id syntax used if specifying a list of properties to uniqueify on, e.g.:
  • For default scene.queryFeatures({ unique: true }), feature.id will be included in uniqueness check
  • For explicit unique property list scene.queryFeatures({ unique: ['$id'] }), feature.id can be included in uniqueness check with $id (un-prefixed names refer to feature.properties)

[bcamper]

@matteblair @tallytalwar I think the most salient thing here for ES is addition of $id syntax for JS functions (which was the most natural way I could imagine access for this, by extending the $ syntax for other variables outside of feature.properties).

[bcamper]

Discussed in chat.

Updated to ensure $id works in filter objects (e.g. filter: { $id: 5 }) in addition to filter JS functions (filter: function(){ return $id == 5; }).