Update the myst-theme to React 19

[choldgraf]

We currently run on an older version of React - we've discussed several times the idea that we should upgrade to version 19, and a few times others have said "we'd be able to do X if we upgrade to React 19". In the name of making sure our priorities are rooted in solving problems either for our users or for ourselves, I'm creating this issue to track this overall effort and the things it might unlock. (or, it'll just live as a little stub that gets closed once we merge #634 who knows 😅)

Things we've be able to do better if we had React 19

• @stefanv said we'd be able to use React-Router to more naturally handle URL redirects without hacky HTML generation

References

• We have a PR that takes a stab at this here: 📦 Move to React 19 and update peerDependencies #634

[stefanv]

Yes, I believe React 19 will make the port from Remix to React Router feasible, which will in turn unlock exporting without a scraper.