React-native-mapbox-gl: Get the current map zoom level.

[mtt87]

I have a use case where I wanted to press a cluster and zoom the map by (currentZoom + 2) but I realised that it's not possible at the moment to retrieve the current zoomLevel.

mapbox-gl-js has this method for example:
https://www.mapbox.com/mapbox-gl-js/api/#map#getzoom

It would be great to have it, probably on the ref this.map.getZoom()

[nitaliano]

getZoom support has been merged into master