GeoJSON and VectorTile types

[jfirebaugh]

Includes some code changes to support the type annotations:

• GeoJSONWrapper implements the VectorTile interface as well as the VectorTileLayer interface. This eliminates special _geojsonTileLayer cases from GeoJSONWorkerSource and WorkerTile.
• Use composition instead of augmenting vector tile objects with rawData, expires, and cacheControl properties. (We still augment individual features with index and sourceLayerIndex; this should be cleaned up in a subsequent refactor.)

[anandthakker]

😍

[anandthakker]

GeoJSONWrapper implements the VectorTile interface as well as the VectorTileLayer interface. This eliminates special _geojsonTileLayer cases from GeoJSONWorkerSource and WorkerTile.

Use composition instead of augmenting vector tile objects with rawData, expires, and cacheControl properties. (We still augment individual features with index and sourceLayerIndex; this should be cleaned up in a subsequent refactor.)

💯