feat: add Web Worker support for marker clustering

[samithahansaka]

Summary

• Adds useSuperclusterWorker hook for offloading clustering to Web Worker
• Adds useMapViewport hook for tracking map bounds and zoom
• Adds worker-marker-clustering example demonstrating 10k+ markers

Problem

Addresses #526 - clustering with large datasets (7,500+ markers) blocks the main thread, causing UI freezes up to 17 seconds.

Solution

Moves Supercluster operations to a Web Worker, keeping the main thread responsive regardless of dataset size.

Changes

• src/hooks/use-supercluster-worker.ts - New hook
• src/hooks/use-map-viewport.ts - New hook
• src/index.ts - Export new hooks
• examples/worker-marker-clustering/ - Full working example

[usefulthink]

I haven't had time to review it, but most importantly: Awesome, thanks a lot for the work!

This is something I wanted to do for a while and never got around to. Looking forward to review and test it tomorrow.

[samithahansaka]

happy to collaborate for a good cause.

[samithahansaka]

@usefulthink any update on this?

[usefulthink]

I now had the time to have a quick look, and it's really nicely done, focused on the important parts and all that.

When I ran it locally with npm run start-local, it didn't work out of the box (there seems to be some styles missing – map div has height: 0 initially). But that should be easy to fix. Also we could do a bit polishing by using a reduced map style and things like that. I can look into that if you like me to.

It still feels a bit slow sometimes, since even with clustering it's rendering too many AdvancedMarkerElements (>800) at once. That could be fixed by increasing the clustering radius a bit.

The only thing that would be important to change is to move the two hooks into the examples src directory instead of the main library. I really like the Idea to export a useMapViewport hook, and I would love to add that as a feature, but I'd like to give that another spin first (add things like configurable events (update instantly when the view changes vs. update on idle), debouncing to avoid emitting too many events, ...).

However, we will likely not add the supercluster-specific hook to the library for the time being, since that is a bit too specific to a certain use-case.

Finally, it still has to be added to the examples-page on the website (there's some documentation about that here, but I just noticed that that page is outdated – examples with external dependencies do work completely fine now).

[samithahansaka]

@usefulthink updated the pr, could you please have a look and let me know if you want more changes.