Adapter doesn't export "handler" like the other adapters

[francoislg]

@sveltekit/adapter-node, svelte-adapter-deno, adapter-node-ws (and probably others) expose their server middleware in an handler.js file when generating the build to allow devs to include this middleware instead of using the full server.

It is particularly useful to merge a storybook server with the application server into a single application, which is what I am trying to do here, and it prevents me from using this adapter.

It would be nice to have the same build output structure.