✨ feat: Add CI workflow, improve build, and update docs

[chriskyfung]

This pull request enhances the project's development infrastructure and maintainability. It introduces an automated CI pipeline, streamlines the build process, and adds comprehensive documentation for users and contributors.

Key Changes

👷 Continuous Integration & Build

• CI Workflow: A new GitHub Actions workflow (.github/workflows/ci.yml) has been added to automate testing and linting on every push and pull request, ensuring code quality and stability.
• Build Process: The package.json scripts have been updated for a more robust and efficient build process.
• Deployment Fix: appsscript.json is now tracked by Git. This is critical for ensuring that clasp deployments have access to the correct manifest file.

📝 Documentation

• User & Developer Guides: New documentation has been added in the docs/ directory, providing clear instructions on both using the script (usage.md) and contributing to its development (development.md).
• Contribution Guidelines: GEMINI.md has been updated with more detailed instructions and examples for using the GitMoji commit convention, making contributions more consistent.

♻️ Refactoring

• Code Cleanup: The unused subtractYears function has been removed from src/code.js to simplify the codebase.