Upgrade to codemirror 6

[benmccann]

I got it partially done, but haven't had time to finish. I would love some help

Here's the work-in-progress: https://github.com/sveltejs/sites/tree/codemirror-6

Please feel free to send PRs against this branch

A couple things to do:

• there's a race condition upon page load
• when the page does load there's an error in the console eventhough things seem to work
• the theme needs to be ported. it's currently in theme.css, but the way themes are created has changed

Alternatively, we could also try replacing the whole REPL with sandpack or web containers, but I think we'd have a harder time using npm dependencies and startup may be slower.

[benmccann]

@PuruVJ you also took a stab at this, right? It sounds like you got further than I did

You mentioned handlebars mode being the only thing you didn't figure out. Perhaps https://codemirror.net/examples/mixed-language/ would be a useful reference as it seems to address that

[benmccann]

learn.svelte.dev now uses codemirror 6! https://github.com/sveltejs/learn.svelte.dev/search?q=codemirror