Improve 'spoof-css ' — mock 'checkVisibility()' method

Steps to reproduce:
1. Add to user rules:
```adblock
example.org##h1
example.org#%#//scriptlet('spoof-css', 'h1', 'display', 'block')
```
2. Go to - https://example.org/
3. In console run:
```js
document.querySelector('h1').checkVisibility()
```

It returns `false` if element is not visible, so we could mock `checkVisibility` to fix it.
https://developer.mozilla.org/en-US/docs/Web/API/Element/checkVisibility