Rename `App`

[Rich-Harris]

Describe the problem

Internally, we have two things called App — the class that is created by svelte-kit build, and instantiated by adapters...

import { App } from 'APP';
import { manifest } from 'MANIFEST';

const app = new App(manifest);

// later
const response = await app.render(request);

...and the namespace for specifying app-level interfaces like locals:

declare namespace App {
  interface Locals {}
  interface Platform {}
  interface Session {}
  interface Stuff {}
}

This hasn't really been a problem, but now that we're adding docs for the types (#4011), it's going to become a source of confusion and broken links.

Describe the proposed solution

I think it makes more sense to rename the class than the namespace. The best suggestion so far is Server:

// created in build_server.js
export class Server {
  constructor(manifest) {
    // ...
  }
}

// created by an adapter
import { Server } from 'SERVER';
import { manifest } from 'MANIFEST';

const server = new Server(manifest);

// later
const response = await server.render(request);

At the risk of inviting bikeshedding, it might also make sense to change server.render to server.respond or server.handle or something, since 'render' sort of implies something side-effect free (which PUT/POST/DELETE requests aren't), and also implies that we're doing SSR when in some cases we're just rendering an empty shell page.

Aside: I've been meaning to overhaul Router and Renderer — having two separate classes seemed like a good idea at the time, but in practice it hasn't worked out, and it would make sense to unify them into a single class. There'd be a nice symmetry if we had a class Server and a class Client.

Alternatives considered

We could rename the namespace instead, though it's less clear what we could rename it to, and it would be a breaking change for more people (all Kit users rather than just adapter authors).

Importance

would make my life easier

Additional Information

No response

[mrkishi]

server.respond() seems appropriate as it returns a Promise<Response>. :)

There'd be a nice symmetry if we had a class Server and a class Client.

It'd also be nice to have the server in a runtime/server/server.js with built data in the manifest à la the client. The less generated code, the easier to navigate the codebase, imo, unless strictly necessary.