gltfpack: glTF 2.1 support

Khronos Group is working on glTF 2.1 specification: https://github.com/KhronosGroup/glTF/issues/2585; I am not sure if gltfpack will support it in full or just partially, but I'll create a tracking issue that notes potential changes we'd need to make and the complexity based on initial review.

Note: it's too early to actually work on supporting these features; that would need an actual spec. Also all of these would need support in https://github.com/jkuhlmann/cgltf before being added to gltfpack.

Complex Scenes:

• External Assets - probably pretty involved
• Bounding Volume Hierarchy - fairly straightforward after cgltf support
• Shapes - fairly straightforward after cgltf support
• Packaging External Assets - similar to "external assets"
• Unified File References - might be as simple as preserving these, not sure
• Multiple Scene Deprecation - can be implemented as a gltfpack warning; we already have some limitations wrt multiple scenes around instancing anyway.

Promoted Extension Functionality:

• EXT_texture_webp - already supported as an extension, might need minor tweaks for promotion
• KHR_materials_emissive_strength - already supported as an extension, might need minor tweaks for promotion
• KHR_mesh_quantization - already supported as an extension, might need minor tweaks for promotion?
• KHR_node_visibility - very straightforward once cgltf support is there

Quality of Life:

• Thumbnails - very straightforward with cgltf support (just preservation of existing thumbnail)
• 64-Bit Binary File Format - should be fairly straightforward with cgltf support
• Non-Sequential Attributes - already supported by gltfpack I believe
• Accessor Component Type Definitions - very straightforward with cgltf support
• Unique IDs - should be fairly straightforward with cgltf support (similar to extras, we could also expose a variant of -kn that is keyed on unique id and not name)