Implement Observer (Listener) Pattern for Online Marketplace Events Using NodeJS

[EpitomeZied]

This pull request introduces a new implementation of the Observer (Listener) design pattern for an online marketplace scenario. It adds a set of classes to model publishers, subscribers, events, and domain entities, enabling decoupled notifications for new products, offers, and job openings. Additionally, it adds a runnable example and documentation to demonstrate the new pattern.

Key changes include:

Observer Pattern Implementation:

Added the OnlineMarketplace publisher, which manages subscriptions and notifies listeners for different event types.
Added the Subscriber base class and concrete subscribers: Customer, JobFinder, and ShippingCompany, each with custom notification behavior.
Added EventType to define supported notification topics (NEW_PRODUCT, NEW_OFFER, JOB_OPENING).
Added Product and Offer domain entities used in published events.

Example and Documentation Updates:

Added main.js to demonstrate subscribing observers and triggering events.
Added Pattern.md with a Mermaid class diagram documenting the design.