[Long Tasks API] Updated the explainer to enable Long Tasks API to work in Web Workers (#906)

* Update explainer for extending the Long Tasks API

Major changes:
- Expanded the goals to include detailed information about the worker type, source URL, and ID
- Improved the problem statement to highlight the lack of support for Long Tasks API in Web Workers.
- Added example code demonstrating long tasks in Web Workers and their impact on task queue.

* Add more explanation

* Add withdrawal rationale for "Extending Long Tasks API to Web Workers" explainer

Update the Withdrawn section in README.md to include the reason for withdrawing
the "Extending Long Tasks API to Web Workers" explainer. The note clarifies that
the team is proposing the Delayed Messages API as an alternative approach.

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Co-authored-by: Joone Hur <joonehur@microsoft.com>

Author: joone