Add darwin style code for raster-dem and hillshade

[mollymerp]

Per https://github.com/mapbox/mapbox-gl-native/pull/10642#discussion_r163034806

Unfortunately, darwin-style-code is unable to automate the entire process of adding a new layer type, so some changes are still needed before this layer type can be used on iOS and macOS:

After #10642 ships we will need to:

• [x] Implement an MGLRasterDEMSource class
• [x] Add the new files to the SDKs Otherwise the new MGLStyleLayer subclass doesn’t get included in the SDK.
• [x] Add the new classes to the documentation table of contents Otherwise the new MGLStyleLayer subclass isn’t listed with the other subclasses in the jazzy documentation.
• [x] Wrap the new layer type in an Objective-C object in the runtime styling API Otherwise the runtime styling API may crash when introspecting the current style.
• [x] Refine the new class’s documentation Otherwise the API reference isn’t self-contained.
• [x] Add an icon to represent this layer type in the sidebar of macosapp Otherwise terrain layers look blank in the sidebar.
• [x] Mention the new APIs in the changelogs Otherwise iOS developers won’t know about the new feature.

[1ec5]

Added some context to each item, for prioritization purposes.