Implement interpreter shutdown

[hoodmane]

This is on top of #2795. It adds an option exitMode which can be set to:

• "no-exit": current behavior.
• "throw": Shut down interpreter then throw an error
• "quit-process": in browser, is the same as "throw". In node, sys.exit(returncode) will exit the node process with the given return code.

"quit-process" will be useful when running pytest.

TODO: close out pyodide API after shutdown.

Checklists

• [ ] Add a CHANGELOG entry
• [ ] Add / update tests
• [ ] Add new / update outdated documentation