Script externalisation doesn't work with fallback

[janopae]

When using the fallback option to generate an SPA (which is probably useful for a lot of extensions), the generated html file will not be rewritten.

This might be because of the order in which things are executed.

It is possible that this can't be fixed on the adapter's side (I'm too inexperienced with SvelteKit to judge). But anyway, I wanted to document this issue.