Implement method bindings for gap/row-gap/column-gap

Summary:
This adds method bindings for `YGNodeStyleSetGap` and `YGNodeStyleGetGap`.

Changelog:
[Genral][Added] - Implement method bindings for yoga gap/row-gap/column-gap

Reviewed By: yungsters

Differential Revision: D39922411

fbshipit-source-id: 6cbb4d352203d2ec92df162c3f2f2efd02bd9568

javascript/sources/nbind.cc

@@ -98,6 +98,8 @@ NBIND_CLASS(Node) {
   method(setPadding);
   method(setPaddingPercent);
 
+  method(setGap);
+
   method(getPositionType);
   method(getPosition);
 
@@ -132,6 +134,8 @@ NBIND_CLASS(Node) {
 
   method(getPadding);
 
+  method(getGap);
+
   method(insertChild);
   method(removeChild);