Add Config::setPointScaleFactor to JavaScript binding

Summary:
Needed for a project I'm working on to properly render on high dpi screens. I've manually tested against my project to confirm it works.
Closes https://github.com/facebook/yoga/pull/612

Differential Revision: D6413517

Pulled By: emilsjolander

fbshipit-source-id: c298d7f6b58dc45aaaede2bb995df8615911a4a9

javascript/sources/Config.hh

@@ -40,6 +40,7 @@ class Config {
  public: // Setters
 
     void setExperimentalFeatureEnabled(int feature, bool enabled);
+    void setPointScaleFactor(float pixelsInPoint);
  
  public: // Getters