Add requires field for root level properties in the spec

Nobody has claimed this yet.

Properties like `terrain` and `fog` are not supported if `projection.name` is anything other than `mercator`. Should documented properties like https://github.com/mapbox/mapbox-gl-js/blob/1333ef17ac2a047d3f7c916566fd078579eb5292/src/style-spec/reference/v8.json#L64-L67

be written with this?

```json
"fog": {
"type": "fog",
"doc": "A global effect that fades layers and markers based on their distance to the camera. The fog can be used to approximate the effect of atmosphere on distant objects and enhance the depth perception of the map when used with terrain or 3D features.",
"requires": [
{
"projection": {
"name": "mercator"
}
}
]
}
```

Start with src/style-spec/reference/v8.json around lines 64-67 and trace how root-level property requirements are represented and consumed. Determine whether the proposed requires structure should be supported for properties such as fog and terrain, then verify that the documented conditional behavior is enforced wherever the spec is processed.

Written by the indexing model from the issue text.