GLTFLoaderPlugin: add constructor and required name getter

[hybridherbst]

The constructor always gets the parser passed, and the name getter is required.

[Methuselah96]

Thanks for the PR!

The parser is already getting passed via the callback in the register and unregister, so I don't think adding a constructor is correct.

export class GLTFLoader extends Loader<GLTF> {
    // ...
    register(callback: (parser: GLTFParser) => GLTFLoaderPlugin): GLTFLoader;
    unregister(callback: (parser: GLTFParser) => GLTFLoaderPlugin): GLTFLoader;
    // ...
}

[hybridherbst]

Thanks! I think you're right about the constructor, and I changed to the readonly prop.