react+JSX plugin

[bcolloran]

(moving issue from pyodide/pyodide#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

[bcolloran]

Hamilton's plugin is here:
https://raw.githubusercontent.com/hamilton/iodide-jsx/master/docs/evaluate-jsx.js

and here is the example plugin spec chunk

%% plugin
{
  "languageId": "jsx",
  "displayName": "React JSX",
  "codeMirrorMode": "jsx",
  "keybinding": "x",
  "url": "https://raw.githubusercontent.com/hamilton/iodide-jsx/master/docs/evaluate-jsx.js",
  "module": "jsx",
  "evaluator": "evaluateJSX",
  "pluginType": "language"
}