GLTFLoader: Add EXT_mesh_gpu_instancing built-in plugin

[takahirox]

Related issue: #21937 From: https://github.com/mrdoob/three.js/pull/24518#issuecomment-1221728569

Description

This PR adds glTF EXT_mesh_gpu_instancing support to GLTFLoader.

I have already made an external EXT_mesh_gpu_instancing plugin. As in #21937 Three.js devs seem to think it's good that GLTFLoader has built-in EXT_mesh_gpu_instancing plugin. So I made this PR based on my plugin.

Demo: https://raw.githack.com/takahirox/three.js/cf5bf1de939d9df3bb26f4923208b17a1723125c/examples/webgl_loader_gltf_instancing.html

Additional context

@Mugen87 I use assets from @shantzis1962 in the example. Please add @shantzis1962 to the change log if this PR lands.