Refactor RMB converter into reusable Rust library

[bigtomcat6]

Summary

• Refactor the project into a reusable pure Rust library with optional N-API bindings.
• Add deterministic money APIs based on integer cents and string parsing.
• Add RmbError for invalid, negative, too-large, and non-finite inputs.
• Preserve the existing rmbToRmb(number) Node.js API for compatibility and add string-based N-API entry points.
• Update README and add GitHub Actions CI for formatting, clippy, Rust tests, and N-API feature checks.

Validation

• Added Rust unit tests covering zero, integer amounts, cents, group boundaries, maximum value, decimal parsing, cents-string parsing, and invalid inputs.
• CI is configured to run cargo fmt --all -- --check, cargo clippy --all-targets -- -D warnings, cargo test, and cargo test --features napi.

Note: local test execution was not available in the current environment, so the PR CI is the source of truth for final validation.