Fixes and Improvements for WASM REPL

[adqm]

Feature or enhancement

Proposal:

I recently ran into some issues with building and using the demo WASM REPL in Tools/wasm/emscripten/web_example/ using the instructions from this page, so I'm proposing some fixes and changes here, specifically:

• Fixes:
  • Exporting HEAPU32 at build time so that python.worker.mjs has access to it to determine the version number
  • Properly closing a <script> tag in python.html so that xterm.js loads properly
• New Features:
  • Augmenting the REPL implementation to support arrow keys for navigation (including accessing history)
  • Resetting things after a process is stopped so we can cleanly restart multiple times
  • Replacing the plain textarea element with a nicer code editor (Ace)

I made these changes while putting together a web demo for a different feature proposal and figured I'd share them in case they were of interest. I'll put in a PR shortly.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

• Builds on Improved usability of WASM web REPL for testing/development. #97747
• Use new REPL for wasm demo #124621 suggests updating the demo to use PyREPL, which would be a nice change but which I didn't attempt to implement yet since it requires much bigger changes to the demo

Linked PRs

• gh-136251: Improvements to WASM demo REPL #136252