Create Unreal coding / style guide

[j9liu]

Distinct from #1540, it might be useful to explicitly state how we write code in the context of Unreal Engine.

There are some Unreal conventions that deviate from our cesium-native style guide, e.g.,

• Prefixes on names, e.g. bMyBoolean, FMyStruct, UMyComponent, AMyActor
• Variable naming differences, e.g. not prefixing pointers with p.

Other knowledge that would be helpful:

• When to use raw pointers vs. SoftObjectPtr, etc. Also, is there any reason we'd use TUniquePtr<T> vs. std::unique_ptr?
• Differences between Unreal's math system vs. glm. For example, FMatrix makes some pretty big assumptions that affect matrix multiplication, whereas glm is more reliable
• Using Unreal containers (e.g., TArray, TMap) for Blueprints or for UI. Other than that, any preference for using vs. std containers?
• Since RTTI is disabled for Unreal, how does Unreal's casting system work?
• Any other pain points we have discovered :)