This PR adds hookpoint for node in GLTFLoader and moves the lights extension handler to the new system.

Background of this PR is #19144 (comment)

In #19144, we use invokeOne() for loadXxx(). But it doesn't fit to Light extension. A node can have mesh, camera, and light extension at a time and all the objects should be created under the node. invokeOne() doesn't fit to it because invokeOne() stops to call the core loadXxx() if extension's loadXxx() creates an object. Then mesh and camera objects will not be created.

A solution idea is adding invokeAll() hookpoint here

GLTFParser.prototype.loadNode = function ( nodeIndex ) {
    ...
    var nodeDef = json.nodes[ nodeIndex ];
    return ( function () {
        var pending = [];
        if ( nodeDef.mesh !== undefined ) {
            pending.push( parser.getDependency( 'mesh', nodeDef.mesh ).then( function ( mesh ) {
                ...
            } ) );
        }
        if ( nodeDef.camera !== undefined ) {
            pending.push( parser.getDependency( 'camera', nodeDef.camera ).then( function ( camera ) {
                ...
            } ) );
        }

        // Add hookpoint here
        parser._invokeAll(ext => ext.createNode && ext.createNode(nodeIndex))
            .forEach(promise => pending.push(promise));

        ...

It is a hookpoint where extension handlers create their special objects in addition to core spec mesh and camera. A good thing is the latter transform and node group initializations can be reused. It very fits to the light extension and @feiss 's text extension.

What do you think of?

Some notes.

• This new hook point can't be used for extensions which want to suppress creating core spec mesh or camera in a node. If users request a hookpoint which can suppress them, we may need to make another hookpoint (maybe loadNode() hookpoint with invokeOne() as loadMaterial, loadTexture, or others does).
• I named createNode for the new hookpoint. Better name idea is welcome. (I know I'm bad at naming in English...)
• I realized that extension handlers which use the new hookpoint for creating their special objects need to manage their NodeRef then markDef() also needs to be extended and the handlers need to call get/addNodeRef().