feat: support mock withImplementation

[9aoy]

Summary

• withImplementation: Accepts a function which should be temporarily used as the implementation of the mock while the callback is being executed.

test('test', () => {
  const mock = rstest.fn(() => 'outside callback');

  mock.withImplementation(
    () => 'inside callback',
    () => {
      mock(); // 'inside callback'
    },
  );

  mock(); // 'outside callback'
});

Related Links

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).