@std/expect: Align `fn` function signature with Jest's `fn` type

**Is your feature request related to a problem? Please describe.**

The current `fn` signature in the Deno std library is:

```typescript
function fn(...stubs: Function[]): Function
```

This lacks type safety and flexibility compared to Jest's `fn`:

```typescript
fn(implementation?: T): Mock;
```

See https://github.com/jestjs/jest/blob/main/packages/jest-mock/src/index.ts#L1082

It makes mocking less intuitive and harder to work with in a type-safe way.

**Describe the solution you'd like**

Update the `fn` function signature to match [Jest's API](https://github.com/jestjs/jest/blob/main/packages/jest-mock/src/index.ts#L1082) for better type inference and usability:

```typescript
fn(implementation?: T): Mock;
```

**Describe alternatives you've considered**

- Keeping the current implementation: This is simpler but sacrifices type safety and consistency with Jest.
- Writing custom wrappers: This adds unnecessary complexity and redundancy for users.

---

I would be happy to contribute this change myself and submit a pull request to update the implementation and associated documentation.