Auto Generated `+server.ts` types for API calling

[AlbertMarashi]

Something which kinda really sucks with creating APIs using SvelteKit +server.ts routes is that all the requests using fetch are untyped, as a result we don't receive typing on the front end, and have to manually constrain types

This can create a lot of surface area for bugs, as there can be a mismatch between the +server.ts and the fetch

My ideas are to somehow get SvelteKit to auto generate these types for use in the frontend

/api/hello/+server.ts

import { APIResponse, json } from "@svelte/kit"
export async function GET():  APIResponse<{ hello: "world" }>{
   return json({
      hello: "world" // these types are typechecked against the function response type to ensure correct data
   })
}

svelte code

export function json<T>(json: T): APIResponse<T> {
   // ...
}

/+page.ts

import { api_fetch } from "@svelte/kit" // a new tool exported by sveltekit that contains autogenerated types for all of the `+server.ts` routes
export async function load() {
   return {
        some_data: await api_fetch("GET", "/api/hello") // returns typed data
   }
   // the return type would look like { some_data: { hello: "world" } }
}

We could imagine that api_fetch looks like this using typescript function overloading with auto generated types

async function api_fetch(method: "GET", "/api/hello"): APIResponse<{ hello: "world" }>;
async function api_fetch(method: "GET", "/api/foo"): APIResponse<{ foo: "bar" }>;
async function api_fetch(method: Methods, url: string): APIResponse<any> {
   // ...
}

I think a lot of this should be possible with some typescript magic similar to how we're already doing with the load functions & page data

1. How should parameterized routes work

Originally posted by @AlbertMarashi in #11042

[teemingc]

Closed as duplicate of #647