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/py-unit/test_pyscript.py

@@ -2,8 +2,9 @@ import sys
 import textwrap
 from unittest.mock import Mock
 
+import js
 import pyscript
-from pyscript import HTML, Element, _html
+from pyscript import HTML, Element
 from pyscript._deprecated_globals import DeprecatedGlobal
 from pyscript._internal import set_version_info, uses_top_level_await
 from pyscript._mime import format_mime
@@ -19,7 +20,7 @@ class TestElement:
         document = Mock()
         call_result = "some_result"
         document.querySelector = Mock(return_value=call_result)
-        monkeypatch.setattr(_html, "document", document)
+        monkeypatch.setattr(js, "document", document)
         assert not el._element
         real_element = el.element
         assert real_element