The implementation here is pretty simple: just give hook merging some custom
logic so it either merges OR resets to an empty array. Matches what was
discussed in the related ticket.

Documentation has been updated to describe this behavior as well.

The dedicated test is just a modified copy of the existing `ky.extend()` one.
Admittedly, I did modify a couple of other tests to cover the edge cases
instead of building brand new tests. No reason other than noticing they were
already close to what I needed and I'm being lazy.

Closes #408

…le-retry-after-statuses into kesills-better-extend-hooks

Manual resolutions required:

- Conflicts between my updated `extend()` tests in `test/main.ts`
  and the tests added with the new function form of `extend()`. Resolved by
  accepting their modified base test and just adding my changes, then putting my
  new `extend()` test below all their new cases.
- Trivial resolution for import formatting in `source/core/Ky.ts`.
- A change to `newHookValue` in `source/utils/merge.ts`, using
  `Object.hasOwn` per the linter's suggestion.