Modernise dev environment & general upgrades

[msxavi]

This pull request introduces several improvements focused on tooling, documentation, and compatibility for the Exportling Rails engine. The most notable updates include adding CI coverage for multiple Ruby versions, providing detailed AI coding guidelines, improving development environment setup, updating dependencies for modern Ruby and Rails, and ensuring serialization compatibility with Rails 8+. These changes enhance maintainability, onboarding, and reliability for both contributors and users.

Tooling and Documentation Improvements

• Add a GitHub Actions workflow (.github/workflows/ruby.yml) to run RSpec tests across Ruby versions 3.0–3.4, ensuring compatibility and continuous integration coverage.
• Introduce an AI coding guidelines document in .github/copilot-instructions.md, detailing Patches's architecture, core components, and required configuration for host apps.
• Add a development container configuration (.devcontainer/devcontainer.json) to streamline onboarding and standardize development environments.

Dependency and Compatibility Updates

• Update patches.gemspec and Gemfile to require Ruby >= 3.0, modernized development dependencies (switching from factory_girl_rails to factory_bot_rails),

Rails 8+ and Serialization Compatibility

• Ensure compatibility with Rails 7 to 8