JSX + React

[mbostock]

Extracted from #1216, this implements client-side support for JSX with React. It looks like this:

```jsx
import {Counter} from "./components/Counter.js";

display(<Counter title="Hello, JSX" />);
```

In JSX blocks, you can only display one thing at a time; subsequent calls to display will replace whatever was displayed previously. Also, you can only display React elements.

TODO:

• JSX code blocks
• JSX modules
• React built-in
• ReactDOM built-in
• Handle syntax errors during JSX transpilation
• Implicit (singular) createRoot within display
• Documentation
• Tests

Future work:

• Allow JSX in inline expressions ${<Component />} (requires acorn-jsx)?
• Allow JSX in Markdown directly (à la MDX)?

Fixes #971.

[Fil]

<OK />!

[mbostock]

Hmm, I found a bug where if you move a JSX block to a new location during preview, it doesn’t get relocated (because we do the special treatment for root._nodes but that doesn’t apply to root._root). I think/hope I can fix that pretty easily though.

Edit: Fixed, but still working on the documentation and then I’ll push up some commits.