Adding missing declarations for serializers

The file `babylon.glTF2Interface.d.ts` from the dist folder is being referenced during development, but filtered out when the package is built.
I am now using the extraDeclarations feature (see last commit) to append it to the entire party.
A different solution would be to manually add the missing dependencies (and removing the `///<reference `  during development )

Tools/Gulp/config.json

@@ -1516,6 +1516,9 @@
                     "../../serializers/src/glTF/2.0/babylon.glTFData.ts",
                     "../../serializers/src/glTF/2.0/babylon.glTFMaterial.ts"
                 ],
+                "extraDeclarations": [
+                    "../../dist/babylon.glTF2Interface.d.ts"
+                ],
                 "output": "babylon.glTF2Serializer.js"
             }
         ],