Prevent translation of code snippets in REPL examples

[Rich-Harris]

We need to add attributes to prevent browsers from translating code snippets when the rest of the page is translated, per this screenshot:

(Obviously we also need to internationalise the site but this is a useful measure in the meantime)

[Conduitry]

https://github.com/sveltejs/svelte-repl/pull/113 added notranslate to a div containing the CodeMirror editor, but I think there are still other places this should happen, like read-only code blocks.