[feat] chaining SvelteKit apps

Describe the problem

The ssr middleware outputted by adapter-node is not a proper middleware because it never calls next(), making it impossible to compose it with other middlewares.

Describe the proposed solution

I'll submit a PR that'll introduce a new build option, handle404?: boolean:

• when set to true (or undefined), handler will handle 404 errors and display a custom error page
• when set to false, handler will call the next middleware on 404

Alternatives considered

Make handler a pure middleware, and handle 404 in index.js, after handler:

https://github.com/sveltejs/kit/blob/668b0aaa2b5626e8f610f395ac551f98eafbd10b/packages/adapter-node/src/index.js#L11-L16

It represents a significant amount of work but allows small performance improvements when using only handler.

Importance

i cannot use SvelteKit without it

Additional Information

No response

Start with the adapter-node SSR middleware and the referenced packages/adapter-node/src/index.js entry point. Trace how handler currently handles 404 responses and how middleware composition would work. Done means the new handle404 option has the proposed default and false behavior, with tests covering both cases.