Type resulting object properties correctly when using change-case/keys

When using `change-case/keys` I would love for the resulting object to be typed correctly without having to do an explicit cast. Currently all transformations produce `unknown`.

```ts
import { camelCase } from "change-case/keys"
import type { CamelCasedProperties } from 'type-fest';

const snake_cased = { foo_bar: 1 }

// Preferably the cast could be left off on the line below
const camelCased = camelCase(snake_cased) as CamelCasedProperties

console.log(camelCased.fooBar) // => 1
```

Start by inspecting the change-case/keys entry point and its TypeScript type declarations. Reproduce the issue with the provided snake_cased example, then trace the existing typing and related tests. Done means the transformed result exposes fooBar without an explicit cast while preserving current behavior.

Written by the indexing model from the issue text.