feat: introduce zig build-backend

[tdejager]

This introduces the zig build backend, that allows you to for now build really simple zig projects. There is some documentation that should explain most of the features.

Let me know if you want a test-suite PR before or after this is merged :)

AI Disclosure

• The implementation was build 50/50 by Claude and myself.
• The documentation and github workflow was generated by AI, which I reviewed.

[Hofer-Julian]

This looks great, Tim. I took the liberty of moving non-parameterized snapshot tests to inline snapshots, as I consider them more readable.

Can you share your example workspace so I can test it locally?

[tdejager]

Hey Julian, thanks for taking a look already. I pushed my changes to: https://github.com/tdejager/pixi-zig-sdl and that should work with an overriden build_backend.