Move pyodide to a web worker (#1333)

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>

pyscriptjs/tests/integration/test_zz_examples.py

@@ -8,9 +8,10 @@ import numpy as np
 import pytest
 from PIL import Image
 
-from .support import ROOT, PyScriptTest, wait_for_render
+from .support import ROOT, PyScriptTest, wait_for_render, with_execution_thread
 
 
+@with_execution_thread(None)
 @pytest.mark.usefixtures("chdir")
 class TestExamples(PyScriptTest):
     """