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:

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

Future work:

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

Fixes #971.

[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.