Code-split SSR routes

[geoffrich]

Follow up to #6. The SvelteKit PR linked in that issue also added code-splitting of server routes, in addition to overhauling the adapter API. It seems like code splitting is only implemented for the Netlify adapter at time of writing, but it would be nice to have this in Azure as well.

The adapter also takes a split configuration setting for the adapter to configure if the app is code-split or not. I believe this is due to the edge case of the adapter falling back to HTTP if the route is not included in the function.

This will introduce a lot of complexity in the generated route config, but could lead to smaller individual Azure functions.

Netlify adapter implementation: https://github.com/sveltejs/kit/blob/f89fb524f851b794cceba4a91864d127dc3948ee/packages/adapter-netlify/index.js#L70