Add support for MOZ_lightmaps and Blender 2.83

This adds support for MOZ_lightmap exporting and upgrades to be compatible with Blender 2.83 (it is NOT backwards compatible with 2.82, or more specifically the GLTF addon that ships with 2.82)

To export a lightmap, create a MOZ_lightamp node from the Add > Hubs menu and hook up a image texture to the Lightmap input. Use a UV Map node to control what UV set should be used for the lightmap, as you would any other texture in Blender.

Note that fore use in Hubs, you currently MUST use the second UV set, as ThreeJS is currently hardcoded to use that for lightmaps. This will likely be fixed in the future so the exporter does not enforce this.