feat(config): support optional private module field

[hackwaly]

• Related issues: None
• PR kind: Feature

Summary

• Support an optional boolean private field in moon.mod, both as a top-level assignment and through options(...), and preserve its value in legacy JSON output. Omission or false denotes public access; true denotes private access in a registry that supports it. No default field is inserted.
• Diagnose non-boolean values and duplicate private declarations, including duplicates across top-level assignments and options.
• Validate the field in both source parsing and validation of an already post-processed AST, without changing public API signatures.
• Add focused regression tests and document the field's semantics.

This PR is limited to private configuration support. It does not add dependencies, change mq command behavior, add runtime entry points, or implement registry authorization or private publishing.

Verification

• moon check --deny-warn
• moon info --target all
• moon fmt --check moon_config
• moon test --target all moon_config: 64 tests passed on each of wasm, wasm-gc, JS, and native

Metadata

• Tests added/updated for bug fixes or new features
• Compatible with Windows/Linux/macOS (local validation on macOS; Windows/Linux not run locally)