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

[AdamWr]

Steps to reproduce:

1. Add to user rules:

example.org##h1
example.org#%#//scriptlet('spoof-css', 'h1', 'display', 'block')

2. Go to - https://example.org/
3. In console run:

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