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

[PutziSan]

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

The current fn signature in the Deno std library is:

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

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

fn<T extends FunctionLike = UnknownFunction>(implementation?: T): Mock<T>;

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 for better type inference and usability:

fn<T extends FunctionLike = UnknownFunction>(implementation?: T): Mock<T>;

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.

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

[andrewthauer]

I fully support this. The current implementation of typing fn to Function requires constant casting and also goes against official TS recommendation to avoid using Function as a type in any interfaces.