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.cpp

@@ -0,0 +1,19 @@
+/**
+ * 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.
+ */
+
+#include "YGConfig.h"
+
+const std::array<bool, YGExperimentalFeatureCount>
+    kYGDefaultExperimentalFeatures = {{false}};
+
+YGConfig::YGConfig(YGLogger logger)
+    : experimentalFeatures(kYGDefaultExperimentalFeatures),
+      useWebDefaults(false),
+      useLegacyStretchBehaviour(false),
+      shouldDiffLayoutWithoutLegacyStretchBehaviour(false),
+      pointScaleFactor(1.0f), logger(logger), cloneNodeCallback(nullptr),
+      context(nullptr) {}