Preprocessor directive defines for Xenko and platforms

Godot recently merged this pull request which allows end-user code to have preprocessor directives to check if the code is running in Godot or if it's on a certain platform. For example, I can do this:

#if GODOT
GD.Print("This is Godot");
#else
throw new InvalidWorkflowException("Only Godot is supported");
#endif

With this feature, it is possible to write code that can run on multiple engines, by enabling/disabling different code depending on the engine. I would like to do something like this:

#if GODOT
GD.Print("This is Godot.");
#elif XENKO
Log.Info("This is Xenko.");
#else
throw new InvalidWorkflowException("Only Godot and Xenko are supported.");
#endif

Additionally, this kind of feature could be useful for intra-Xenko uses, for example, you could have a define called XENKO_WINDOWS and use that to wrap around any Windows-specific code.

No files, tests, or entry points are named. Start by locating where C# preprocessor symbols are configured for Xenko and platform-specific builds, then define the supported symbols and verify that user code can distinguish engines and platforms as described.