Feature/web worker

[yush-1018]

This is a Draft PR implementing the foundation for the Web Worker API (Worker class) in boa_runtime.

What this PR does

1. Adds a new worker module in core/runtime/src/worker/
2. new Worker("script.js") spawns a fresh OS thread with its own Context and runs the given script file
3. worker.postMessage(msg) sends a string message to the worker thread using Rust's mpsc channels
4. worker.terminate() is stubbed out for now
5. Registered as a WorkerExtension in the runtime, so it's available out of the box
6. Added a basic smoke test

What's missing (and where I'd love some guidance )
This is intentionally a Draft — I wanted to get the architecture right before going further. A few things I'm unsure about:

1. Worker → Main thread messaging — How should onmessage work on the main thread side? Should we poll via run_jobs(), or is there a plan for a proper event loop in boa_runtime?
2. Structured Clone vs simple JSON — Right now I'm only sending strings. Should I go for full Structured Clone, or is a JSON-based approach fine as a starting point?
3. Worker global scope — The worker's Context is pretty bare right now (no console, setTimeout, etc.). What's the best way to bootstrap a DedicatedWorkerGlobalScope without creating circular deps with boa_runtime?
4. terminate() — Should I use engine interrupts to actually kill the worker thread?

Happy to iterate on any of this based on your feedback. Thanks for your time! 😊