Add a new scriptlet — 'trusted-prevent-xhr'

[AdamWr]

Similar to - #350, but instead of using redirection resources which are limited, we could have a trusted scriptlet which will allow to set any content in response/responseText, and maybe allow to set other values as well.
Also generating random content (like in prevent-xhr) would be useful, I think.
Something like (just an example):

example.org#%#//scriptlet('trusted-prevent-xhr', 'pagead2.googlesyndication.com', 'length:100-300, responseContent: ins.adsbygoogle-ablated-ad-slot')

in the case if website will check length of response and if it contains specific text.

[piquark6046]

CC: @gorhill