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>

docs/development/developing.md

@@ -190,6 +190,16 @@ $ pytest test_01_basic.py -k test_pyscript_hello -s --dev
 `--dev` implies `--headed --no-fake-server`. In addition, it also
 automatically open chrome dev tools.
 
+#### To run only main thread or worker tests
+
+By default, we run each test twice: one with `execution_thread = "main"` and
+one with `execution_thread = "worker"`. If you want to run only half of them,
+you can use `-m`:
+
+```
+$ pytest -m main    # run only the tests in the main thread
+$ pytest -m worker  # ron only the tests in the web worker
+```
 
 ## Fake server, HTTP cache