Examples: Modularized PMREM and added webgl2_loader_gltf

[oparisy]

Following discussions with @donmccurdy, this PR provides a fully functional version of the glTF loader example using modules from examples/jsm.

For this purpose I needed to include the examples/js/PMREM utils to the modularize.js script provided by #9562. This PR also provides .d.ts type definitions for those utilities.

Following @mrdoob advice, I named this example webgl2_loader_gltf.html since webgl2 samples already use ES6 syntax. I chose to use a coding style closer to the examples/jsm files though (only import needed modules from three.module.js, no THREE prefix).

I also took special care to minimize differences with the original webgl_loader_gltf.html, so that they can be diffed for reference purpose.

I'd especially appreciate a code review on this coding style, and the provided type definitions.

[donmccurdy]

This should be using WebGL2Renderer rather than WebGLRenderer – I tried making that change but:

1. WebGL2Renderer is not currently exported by the ES module version (easy to fix)
2. WebGL2Renderer is not compatible with PMREMGenerator (??? to fix)

[mrdoob]

I'm hesitant about WebGL2Renderer. At this point, seems like it'll be easier to just make WebGLRenderer use WebGL 2.0 when available. If we're going to create a new renderer from scratch, maybe better to focus on WebGPURenderer instead.

[donmccurdy]

Ah ok. In the meantime, should we hide this example from the sidebar until it uses webgl2, or list it elsewhere?

[mrdoob]

Yeah, we should do some clean up next cycle.

[oparisy]

May I suggest an ES6 or JSM prefix instead of webgl2 then?