Add Event Emitters for Section Events

[bae080311]

Summary

To make the jump-section library more extensible and easier to integrate with other UI components or custom JavaScript logic, developers need a way to react to various section lifecycle events. A robust event emission system will provide this crucial extensibility point.

Proposed Solution

• Implement a simple, efficient event emitter pattern (e.g., on, off, emit) within the core library.
• Emit specific, well-defined events at key lifecycle stages, such as section:enter (when a section enters the viewport), section:leave (when it leaves), section:activate (when it becomes the primary active section), and section:deactivate (when it ceases to be primary active).
• Ensure that event payloads include relevant data, such as the section element, its ID, and potentially the direction of scroll or intersection ratio.

Acceptance Criteria

• The library exposes an API (e.g., instance.on(), instance.off()) for subscribing to and unsubscribing from events.
• section:enter and section:leave events are reliably emitted when a section crosses the viewport boundary.
• section:activate and section:deactivate events are emitted when a section becomes or stops being the designated primary active section.
• Event payloads consistently include the section's DOM element and its ID.
• Comprehensive documentation provides examples of how to utilize the new event emitters.

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