feat: Try out some simple preloading

[rschristian]

The Tutorial & REPL have some pretty rough waterfalls at the moment, too many dynamic imports (both intentional & automatically split by Vite). This should hopefully help.

Whilst we really could do with extracting the routing logic out of the render tree (and this app would actually be a fairly good demo for that sort of thing), for now, it's pretty easy to just tack this on to the few pages that rely on client-side scripts heavily.