Improve RequestHandler and EndpointOutput type declarations

[dannyvelas]

If I wanted my post function to return a number as a response body and I accidentally return a string, there is no way I can let the type-checker prevent situations like these. There is no way I can tell it "hey, make sure the body property is always a number". So, the following typechecks:

export const post: RequestHandler<Locals, MyBody> = async (request) => {
  return {
    headers: { "set-cookie": "jwt=token; Path=/; HttpOnly" },
    body: "wrong type"
  };
};

This PR changes RequestHandler and EndpointOutput so that users can specify their response body type as number by making number the third generic argument to RequestHandler. As a result, this won't typecheck:

export const post: RequestHandler<Locals, MyReqBody, number> = async (request) => {
  return {
    headers: { "set-cookie": "jwt=token; Path=/; HttpOnly" },
    body: "wrong type",
  };
};

I wanted to make the second generic parameter to RequestHander, ReqBody be of default type DefaultBody, but this causes an error in packages/kit/src/runtime/server/endpoint.js on line 27, because the request.body: BaseBody is not type compatible with BaseBody & DefaultBody, which is the type it would end up with (per the type definitions of ServerRequest and ParametrizedBody).

There might be a better way to avoid the error and give ReqBody stronger typing than unknown, but I don't currently know enough about the codebase to do so.

Background for this PR: In the discord channel contribute-to-svelte-kit, I proposed this idea a few messages from the last.

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🦋 Changeset detected

Latest commit: 27d8b34

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[ignatiusmb]

This looks good, just a couple of comments. A changeset would also be nice.

[dannyvelas]

Cool. I just made the changes:

1. Adding extends DefaultBody to the generic parameter of EndpointOutput and the last generic parameter of RequestHandler.
2. Changed generic type names ReqBody and ResBody to Input and Output, respectively, in RequestHandler.
3. Changed generic type name ResBody to Body in EndpointOutput.
4. Added patch changeset, tall-hotels-punch.

Let me know if I misunderstood any of your suggestions - I'd be happy to go back and amend any mistakes.

[ignatiusmb]

Thanks! Everything else looks good to me.