Add JSON generics to ky() and HTTPError

[sholladay]

Closes #615
Closes #485
Relates to #312

This PR adds support for an optional TypeScript generic type parameter in all request methods, passes it through the request, response promise, and response, and into .json(), so that users have more flexibility in where and how they type their JSON objects.

All of the following forms are supported:

• ky<T>()
• ky.get<T>() (all methods)
• ky().json<T>()
• (await ky()).json<T>()

The HTTPError class has similarly been updated and passes its type parameter through to error.response.json(). See #615 for an example of how this could be used.

[nielsdB97]

Looks good! Thanks for your efforts on getting this done 😄

[mfulton26]

This is neat. What do you think of adding a second generic type parameter for the expected Error payload too?

try {
  // user is a User
  const user = await ky<User, UserGetError>('/api/users/2').json();
} catch (exception) {
  if (exception instanceof HTTPError) {
    // error is UserGetError
    const error = await exception.response.json();
  }
}

[sholladay]

Seems like a good idea. 👍

[mfulton26]

Oh, it might already be there, kind of. Plus, I don't think there's a way in TypeScript to propagate a generic type parameter from a method call through to an exception that is thrown and it show up in the catch() block.

I think this works as is though:

try {
    // user is a User
    const user = await ky<User>("/api/users/2").json();
} catch (exception) {
    if (exception instanceof HTTPError) {
        // exception is UserGetError
        const error = await exception.response.json<UserGetError>();
    }
}

In fact, I see that <T = unknown> was added to HTTPError. Is that usable though?

[mfulton26]

I see that <T = unknown> was added to HTTPError. Is that usable though?

I suppose when creating instances manually it is.

[sholladay]

It was added because of #615. See that issue for how and why you might use the generic for HTTPError.

And if TypeScript ever does allow functions to type their exceptions, HTTPError will be ready for it.