Fix compilation on MSVC by moving YGConfig to C++

Summary:
This PR fixes the compilation on MSVC. I moved the `YGConfig` creation to a C++ constructor.

Addionally it removes the "dot" notation on `YGValue`, I didn't want to change that type to a C++ constructor, because I think this will break the ABI.
Closes https://github.com/facebook/yoga/pull/746

Differential Revision: D7498141

Pulled By: emilsjolander

fbshipit-source-id: 5f5308ff838dcd803065785ddc08b2404524acb9

yoga/YGConfig.h

@@ -0,0 +1,23 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+#include "Yoga-internal.h"
+#include "Yoga.h"
+
+struct YGConfig {
+  std::array<bool, YGExperimentalFeatureCount> experimentalFeatures;
+  bool useWebDefaults;
+  bool useLegacyStretchBehaviour;
+  bool shouldDiffLayoutWithoutLegacyStretchBehaviour;
+  float pointScaleFactor;
+  YGLogger logger;
+  YGCloneNodeFunc cloneNodeCallback;
+  void* context;
+
+  YGConfig(YGLogger logger);
+};