RequiredDeep: Fix behaviour with undefined

[som-sm]

Fixes #1217

RequiredDeep currently removes all undefineds, so RequiredDeep<{a: string | undefined}> returns {a: string}, even though a is not optional. But IMO, it should only affect optional properties similar to how the built-in Required utility works.

If there's a request, we can consider adding an option to remove undefineds as well, but that shouldn't be the default behaviour.

This PR:

• Refactors RequiredDeep to only operate on optional properties.
• Removes all the existing tests because most of the current cases have | undefined attached to them.
• Adds more test cases, improving the coverage.

[sindresorhus]

Looks good. Thanks for fixing 👍

Hope you had a great summer.

[som-sm]

I did! Hope yours was great as well 🙂