glTFSerializer: hide exporter.extensions from public api documentation

serializers/src/glTF/2.0/Extensions/KHR_texture_transform.ts

@@ -1,5 +1,8 @@
 /// <reference path="../../../../../dist/preview release/gltf2Interface/babylon.glTF2Interface.d.ts"/>
 
+/**
+ * @hidden
+ */
 module BABYLON.GLTF2.Exporter.Extensions {
     const NAME = "KHR_texture_transform";
 

serializers/src/glTF/2.0/babylon.glTFExporterExtension.ts

@@ -26,10 +26,4 @@ module BABYLON.GLTF2.Exporter {
 
 
     }
-}
-
-/**
- * Defines the module for the built-in glTF 2.0 exporter extensions.
- */
-module BABYLON.GLTF2.Extensions {
 }