Support symbols for the source._dataType='vector' instead of source.type=='vector'

Nobody has claimed this yet.

## Motivation

So to add support to the custom vector layers I noticed that in the style.js we only support symbol layers for the `vector` and `geojson` source types. However I noticed that there is another parameter `source._dataType` which can be used so it also supports custom source types

Here is the line that checks for the source.type

https://github.com/mapbox/mapbox-gl-js/blob/fd2045fd6aef55d9265c9a4c43fb48b294289482/src/style/style.js#L1408

I guess changing it to check based on `source._dataType` will allow custom sources to support symbols too

## Design Alternatives

it allows to add better support for the vector custom sources.

### Implementation

https://github.com/mapbox/mapbox-gl-js/blob/fd2045fd6aef55d9265c9a4c43fb48b294289482/src/style/style.js#L1408

I guess changing it to check based on `source._dataType` will allow custom sources to support symbols too

Start at src/style/style.js around the source.type check linked in the issue, and trace how symbol-layer support is selected for vector and geojson sources. Verify the change would recognize sources whose _dataType is vector while preserving existing behavior for the built-in source types.

Written by the indexing model from the issue text.