Generate assets that utilize KHR_materials_unlit

There are currently assets generated that use KHR_materials_pbrSpecularGlossiness but not KHR_materials_unlit. It would be useful to properly test support for this extension.

Inspect the asset-generation entry points and existing assets using KHR_materials_pbrSpecularGlossiness. Determine where generated glTF assets are defined, then add coverage for KHR_materials_unlit. Done means the generator produces assets that exercise this extension for importer validation.

Written by the indexing model from the issue text.