Improve 'prevent-fetch' — add ability to set random response content

[AdamWr]

Similar to - #199 (comment) but for prevent-fetch.

Also adding Content-Length header to response would be a good idea, related to - AdguardTeam/AdguardFilters#175797 and AdguardTeam/AdguardFilters#176137

Steps to reproduce:

1. Add this rule:

example.org#%#//scriptlet('prevent-fetch', 'pagead2.googlesyndication.com')

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

(async () => {
    const ads = await fetch('https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js');
    const contentLength = (await ads.text()).length;
    const contentLengthHeader = Number(ads.headers.get('Content-Length'));
    if (!ads.ok || contentLength < 100 || contentLengthHeader < 100) {
        alert('Ads are blocked!');
    }
})();

Alert will be displayed.

[Yuki2718]

Any news on it? japscan now checks if the response has at least 100 characters (they'll change in a few hours). This will also be useful for AdguardTeam/AdguardFilters#201832