Allow extend() to append to prefixUrl about ky

[sindresorhus]

I think something like this would be useful. It should probably be called appendPath to make it clear what it does. I would like to see feedback more people first though.

[sholladay]

I propose a more generic feature to solve this. When using .extend(), pass a function and Ky will provide you the existing options, which you can then modify as you see fit and return. The return value is spread into the existing options, just as though you had passed it directly to .extend() without using a function.

const api = ky.create({ prefixUrl: "https://www.example.com/api" });
const sales = api.extend((options => ({ prefixUrl: options.prefixUrl + "/sales/v1" }));
await sales.post("offers", { json: {} }); // POSTs to https://www.example.com/api/sales/v1/offers

[DCKT]

+1 ! This functionality can be useful when an API has multiples levels.
I also think this should be under a new function to avoid breaking change and explicit behavior.

[mfulton26]

kyInstance.appendPath() could be nice (explicit, simple, no changes to extend())