Allow dynamic index.svelte route (not prerendered)

[geoffrich]

Azure SWA require an index.html file to be present. At the moment, this means your root index.svelte route needs to be prerendered.

We should also support a dynamic index route that is SSR'd at request time. Potential approach:

• Detect if index.html was prerendered. If so, do nothing.
• Otherwise, create an empty index.html to make the build succeed. Update the routing rules to redirect requests for / to the api/__render function instead. I suspect the routing update is needed so Azure doesn't serve the empty HTML file instead.

See also: #2, #4

[geoffrich]

sveltejs/kit#2675 should help with detecting if index.html was prerendered.

[geoffrich]

Blocked by sveltejs/kit#3099.

[geoffrich]

svelte-adapter-azure-swa@0.3.0 no longer requires index.svelte to be prerendered.