Allow Workers w/out SharedArrayBuffer

[WebReflection]

Description

This MR is a follow up integration of pyscript/polyscript#80

Beside fixing the exported JS' PyWorker, which was fully broken and not smoke-tested properly, these changes allow the usage of PyScript in workers without DOM primitives or mechanism that requires synchroonus worker -> main operations, still allowing the worker code to expose, via pyscript.sync, any utility / callback that can be then consumed via the main thread only asynchronously.

Example

<!-- main thread -->
<script type="module">
import { PyWorker } from '@pyscript/core';

const { sync } = await PyWorker(
  './worker.py',
  { config: { sync_main_only: true } }
);

console.log(await sync.utility(3));
// log: 6
</script>

# worker.py
from pyscript import sync

def utility(num):
  # some complex operation
  return num * 2

sync.utility = utility

This MR takes the opportunity to also expose MPWorker beside PyWorker as that was also missing before.

Changes

• breaking (but then again, it was already broken): the PyWorker in the JS side of affairs returns a promise which is resolved when the worker has fully bootstrapped, guaranteeing that sync utilities can be accessed later on
• new: it is now possible to use sync_main_only configuration flag to disable any DOM in workers but still allow the main thread to interact with their exported pyscript.sync utilities
• added a smoke test that cannot be tested with the rest of the tests because the server must bootstrap without any special header
• simplified even more the worker initialization (compared to its primitive polyscript counter part) by awaiting out of the box the lately introduced ready promise for XWorker

Checklist

• All tests pass locally
• I have updated CHANGELOG.md
• I have created documentation for this(if applicable)

[ntoll]