feat(project): load Puff project config

[ofabiodev]

Summary

This PR adds initial Puff project configuration loading.

Changes

• Adds puff.toml config structs, defaults, loading, validation, and nearest-config discovery.
• Adds support for pack, Minecraft, and build configuration sections.
• Adds tests for config loading, defaults, validation errors, and config discovery.

Testing

cd src
go test ./...