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_py_config.py

@@ -6,7 +6,7 @@ from pathlib import Path
 import pytest
 import requests
 
-from .support import PyScriptTest
+from .support import PyScriptTest, with_execution_thread
 
 
 @pytest.fixture
@@ -35,6 +35,15 @@ def unzip(location, extract_to="."):
     file.extractall(path=extract_to)
 
 
+# Disable the main/worker dual testing, for two reasons:
+#
+#   1. the <py-config> logic happens before we start the worker, so there is
+#      no point in running these tests twice
+#
+#   2. the logic to inject execution_thread into <py-config> works only with
+#      plain <py-config> tags, but here we want to test all weird combinations
+#      of config
+@with_execution_thread(None)
 class TestConfig(PyScriptTest):
     def test_py_config_inline(self):
         self.pyscript_run(