VOXLoader: Added scene graph support.

[mrdoob]

Related issue: #27803

Description

Implemented scene graph support based on the spec extension here:
https://github.com/ephtracy/voxel-model/blob/master/MagicaVoxel-file-format-vox-extension.txt

Also changed the loader API.

Before:

const loader = new VOXLoader();
loader.load( 'models/vox/monu10.vox', function ( chunks ) {

	for ( let i = 0; i < chunks.length; i ++ ) {

		const chunk = chunks[ i ];

		const mesh = new VOXMesh( chunk );
		scene.add( mesh );

	}

}

After:

const loader = new VOXLoader();
loader.load( 'models/vox/monu10.vox', function ( result ) {

	scene.add( result.scene );

}