react+JSX plugin

(moving issue from https://github.com/iodide-project/pyodide/issues/626)

Quite some time ago @hamilton built a react+JSX plugin, but it was only intended as an experimental proof-of-concept; I believe it was not polished and QA'ed to Hamilton's satisfaction, and since it was mostly an experiment, it has not been maintained.

Nonetheless, it did work, and so it remains a good starting point. It would probably not take a ton of work to get it working again at the level to which Hamilton had built it out, and work towards making it more robust could proceed from there (though I don't recall at this point what more it might have needed).

cc @Darruma

Start by locating the experimental React+JSX plugin described in this issue and review the original issue #626 for its history. Assess what still works and what was left incomplete, then define the expected robustness and QA scope before implementation. Done means a maintained, working plugin with agreed coverage and validation.

Written by the indexing model from the issue text.