Notify MGLMapViewDelegate when style loading completes

[1ec5]

As of #6371, MGLMapView.mm has a compiler warning due to the new MapChangeDidFinishLoadingStyle signal not being handled by an MGLMapViewDelegate method:

Semantic Issue Group
/Users/mxn/hub/mapbox-gl-native/platform/macos/src/MGLMapView.mm:753:13: Enumeration value 'MapChangeDidFinishLoadingStyle' not handled in switch

We’ll need to add -[MGLMapView mapView:didFinishLoadingStyle:] to both the iOS and macOS SDKs. The second parameter would be an MGLStyle, at least once #6097 lands and an instance of MGLStyle sticks around a bit longer.

Adding this method will take us a little further from the set of delegate methods exposed by MapKit (#2775), but MapKit doesn’t have the concept of remote map types anyhow (at least publicly).

/cc @friedbunny

[1ec5]

A block-based style setter (#6386 (comment)) would be a fine alternative, but we need this delegate method in the meantime.