Feature: a way replace inner function in `spy` mocking

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

In an event emitter, I want to replace the handler to throw an error, to verify if the emitter can handle error

**Describe the solution you'd like**

Like `node:test`, you can replace the mock implementation with
```ts
import { mock } from 'node:test'
mocked_fn = mock.fn()
mocked_fn.mock.mockImplementation(()=>{ /* new logic */ })
```

**Describe alternatives you've considered**

make the `fn` in `spy(fn)` a wrapper, which call an external function variable.

No file or test is named in the issue. Locate the existing spy implementation and its tests, then compare the current API with the node:test mockImplementation example; done means a spy's inner function can be replaced so event-emitter error handling can be tested.

Written by the indexing model from the issue text.