Added:
- Added roundtrip tests for all models in the glTF-Sample-Models repository
- Added "extensionsUsed" and "extensionsRequired" properties to model

Changed:
- Create missing parent directories automatically when exporting the model (and its resources)
- Retain empty strings, lists, and dictionaries in the model when exporting (only remove properties set to None, leave everything else the same)

Fixed:
- Loosened type restrictions on "extensions" property to avoid error during load when the property is set