[Feature] `locator.screenshot()` add option for expanding space captured around locator

Nobody has claimed this yet.

When creating visual regression tests(VRT) for a variety of elements it would be very helpful if the `locator.screenshot()` method had an option to add extra space around the locator.

This is particularly handy for testing `:focus-visible` CSS states that are actually outside the bounding box of the element. It would also be helpful for hover and active states that may go outside the element, likely other scenarios as well.

Example
```js
const button = page.locator('button');
await button.focus();

expect(await button.screenshot({ expandAreaBy: '20px' })).toMatchSnapshot(
'button-focused.png',
);

// Alternative option syntax
expect(await button.screenshot({ expandAreaBy: { value: 20, unit: 'px' } })).toMatchSnapshot(
'button-focused.png',
);
```

Start by locating the locator.screenshot() API and its screenshot implementation, then inspect existing screenshot option tests. Verify how focus-visible, hover, and active visuals can extend beyond the locator bounds, and add coverage showing that the requested expansion syntax captures the surrounding area as expected.

Written by the indexing model from the issue text.