support module workers?

Currently the worker is always created in classic mode. Since firefox now supports module workers, it would be nice to have the option for it in greenlet. It would open up some useful functionality, like dynamic imports.

I can submit a PR if this sounds desirable.

Start by locating where greenlet creates its worker and review the existing worker-related tests. Check how an option could select module versus classic worker behavior, then verify that module workers support the requested dynamic-import use case while classic mode remains unchanged.

Written by the indexing model from the issue text.