Refactor "GeoJSONSource" as a composition "VectorTileSource"

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See the instructions in [this comment](https://github.com/mapbox/mapbox-gl-js/pull/2918#issuecomment-241714024) for a description of what would currently be required to create a custom variation of `VectorTileSource`.

On the worker side, it's not bad at all: `VectorTileWorkerSource` is designed to accept a custom `loadVectorTile` in its constructor. But on the main thread, the whole `VectorTileSource` has to be forked in order to add a `type: 'my-custom-source-type-name'` property to any params sent to the worker (so that messages go to the custom worker source rather than the core VectorTileWorkerSource).

If `VectorTileSource` just kept a `this.type` property from its constructor options and sent it along in all its worker requests, this would simplify a custom source author's job significantly.

cc @lucaswoj

Start with the linked pull request comment, then inspect VectorTileSource and VectorTileWorkerSource to understand how worker requests select a source. The refactor is complete when a custom source can provide its type through worker requests without forking VectorTileSource; identify or add coverage for that behavior if the existing tests expose these entry points.

Written by the indexing model from the issue text.