Next.js bans the use of react-dom/server APIs on the client. React's
docs recommend against using these too:
https://react.dev/reference/react-dom/server/renderToString#removing-rendertostring-from-the-client-code
In this diff, we switch from using `ReactDOMServer.renderToStaticMarkup`
to `ReactDOMClient.createRoot`, fixing SVG exports in next.js apps.
`getSvg` remains deprecated, but we've introduced a new `getSvgElement`
method with a similar API to `getSvgString` - it returns an `{svg,
width, height}` object.
### Change Type
- [x] `sdk` — Changes the tldraw SDK
- [x] `bugfix` — Bug fix
