Add light property to MGLStyle

[1ec5]

Per #8431 (comment), MGLStyle needs a light property that lets the developer set style-wide parameters like lighting color and intensity. We’ll need an MGLLight class to hold these individual properties. Hopefully we can make MGLLight consistent with SceneKit’s SCNLight and the style specification simultaneously.

One complication is that mbgl::Map::setLight() takes a std::unique_ptr instead of an mbgl::style::Light object by value (which is how mbgl::CameraOptions and mbgl::AnimationOptions work). Developers will expect MGLLight to be a value class along the lines of MGLMapCamera.

/cc @fabian-guerra @lbud @jfirebaugh

[ivovandongen]

@fabian-guerra FYI I'm making some slight adjustments to the light API in #8889.

[1ec5]

#8889 moves mbgl’s Light interface to codegen, which suggests that we should probably use codegen to build MGLLight.h and MGLLight.mm as well.

[1ec5]

#8841 is the corresponding Android issue.