feat: improve types with generic extend

[spence-s]

Checklist

• I have read the documentation.
• I have included a pull request description of my changes.
• I have included some tests.
• If it's a new feature, I have included documentation updates in both the README and the types.

This changes the way types work in Got by making got a generic function of the passed in options.

Fixes #1117 and opens up to some new generic patterns that may make the TS experience better. It doesn't look like there are any specific tests for types, but this was light enough change that it didn't take any code changes.

There shouldn't be really any "breaking" changed here, the types work a little different, but that shouldn't be noticeable to most I don't think.

[sindresorhus]

It would be useful with some kind of type test that proves that this change actually works. Could just do some https://github.com/mmkal/expect-type assertions in one of the test files.

[sindresorhus]

Thanks :)