gh-124621: Emscripten: Support pyrepl in browser (GH-136931)

Basic support for pyrepl in Emscripten. Limitations:
* requires JSPI
* no signal handling implemented

As followup work, it would be nice to implement a webworker variant
for when JSPI is not available and proper signal handling.

Because it requires JSPI, it doesn't work in Safari. Firefox requires
setting an experimental flag. All the Chromiums have full support since
May. Until we make it work without JSPI, let's keep the original web_example
around.

Co-authored-by: Łukasz Langa <[email protected]>
Co-authored-by: Éric <[email protected]>