Non standard forward declaration assuming namespace breaks unity builds

[kunitoki]

In some files (nested_bool.cpp nested_trigger.cpp and nested_number.cpp), there are wrong forward declarations:

using namespace rive;
class StateMachineInstance; // < this is not in the global namespace, and results in a different symbol because it's not enclosed in the correct namespace (using namespace here is not placing all symbols in that namespace)

// more code here that should be inside the rive namespace

should be instead

namespace rive {
class StateMachineInstance;

// more code here that should be inside the rive namespace
} // namespace rive

Also consider not doing using namespace rive; in cpp in the global namespace, this is breaking unity builds.