This PR adds support for optionally running pyodide in a web worker:
- add a new option config.execution_thread, which can be `main` or `worker`. The default is `main`
- improve the test machinery so that we run all tests twice, once for `main` and once for `worker`
- add a new esbuild target which builds the code for the worker
The support for workers is not complete and many features are still missing: there are 71 tests which are marked as `@skip_worker`, but we can fix them in subsequent PRs.
The vast majority of tests fail because js.document is unavailable: for it to run transparently, we need the "auto-syncify" feature of synclink.
Co-authored-by: Hood Chatham <roberthoodchatham@gmail.com>
Co-authored-by: Madhur Tandon <20173739+madhur-tandon@users.noreply.github.com>
