Support rendering in Web Workers and Cloudflare Workers

[email protected] fails when rendering ELK-based diagrams inside a browser Web Worker:

TypeError: Cannot set property self of #<WorkerGlobalScope> which has only a getter

Flowchart, State, Class, and ER diagrams trigger the error, while Sequence and XY Charts render successfully.

The issue appears to come from ensureElk() deleting and restoring globalThis.self, which is read-only in WorkerGlobalScope.

It would be helpful to:

• Support rendering in browser Web Workers;
• Ensure compatibility with Cloudflare Workers;
• Add tests for both Worker environments.