ISSUE-10: Implement comment subscription feature with event dispatching

[navneetrai]

Fixes #10

This pull request introduces a new subscription feature to the comment system, allowing users to subscribe to commentable entities and receive notifications when new comments are posted. It adds support for managing subscriptions, dispatching distinct events for subscribers, and configuring notification behavior. The most important changes are grouped below.

Subscription Feature Implementation:

• Added a new CommentSubscription model and corresponding migration to store subscriptions, enabling users to subscribe to any commentable entity. [1] [2]
• Extended the HasComments trait with methods to subscribe, unsubscribe, and retrieve subscribers for a commentable entity.

Event Dispatching and Configuration:

• Introduced the UserIsSubscribedToCommentableEvent event and updated the comment creation logic to dispatch this event to subscribers, with configuration to optionally treat subscribers as mentions instead. [1] [2] [3]
• Updated documentation to reflect new events and configuration options for subscriptions. [1] [2]

Testing and Migration Registration:

• Added comprehensive tests to verify event dispatching for subscribers and registered the new migration in the service provider and test setup. [1] [2] [3]