[glTF] Implement glTF2.0 common materials, clean up loadMaterials().

[donmccurdy]

Changes:

• Implement glTF2.0 KHR_materials_common extension.
• Implement glTF2.0 KHR_lights extension (no longer part of common materials).
• Isolate WebGL technique code in its own extension object.
• Clean up loadMaterials() a bit.
• Update docs.

All currently-supported examples work as before. I don't have official examples for common materials or lights yet, but went ahead and updated the Duck model using the blender extension.

Sorry that the diff is unreadable... loadMaterials needed a bit of un-nesting. The gist of the changes' structure is shown here:

https://github.com/donmccurdy/three.js/blob/fb2066bb899f55d4a9ddf519cf4d0298e99aafca/examples/js/loaders/GLTF2Loader.js#L1914-L1934

/cc @takahirox

[mrdoob]

Looks good!

[mrdoob]

Thanks!