refactor: export gltf parsers to the user to do some custom operations

[zhuxudong]

const customParser =  new GLTFParser([
    CustomParser,
    ......
  ]);

 const customResource = new GLTFResource(engine);
 customResource.json = json;
 // customResource.arraybuffer = arraybuffer;

 customParser.parse(customResource).then((customResource)=>{
    console.log(customResource)
 })