add zig 0.15 support

[tristanpemble]

I did:

• blindly just copy zcgltf.zig into io.zig with basically zero critical thought to address the usingnamespace deprecation
• run zig build test until it passed
• render a cube in my own project using this branch

I did not:

• test this much more than a cube
• use an LLM to do this work

appropriate consideration might be warranted; I just got this to a working state until I can pull it into my own codebase