Support terrain view

[Dan Vanderkam]

Mapbox GL JS 2.2.0 added support for terrain view via a raster-dem source:

map.addSource('mapbox-dem', {
  'type': 'raster-dem',
  'url': 'mapbox://mapbox.mapbox-terrain-dem-v1',
  'tileSize': 512,
  'maxzoom': 14
});
// add the DEM source as a terrain layer with exaggerated height
map.setTerrain({ 'source': 'mapbox-dem', 'exaggeration': 1.5 });

It would be great to have support for this in React-mapbox-gl.