Improve content type detection using Sec-Fetch-Dest header

[ameshkov]

We've already done it once, but I guess it's time to refresh the list of content types we're able to detect: https://www.w3.org/TR/fetch-metadata/#sec-fetch-dest-header

Additionally, we can now easily distinguish iframes (Chrome sends Sec-Fetch-Dest: iframe, we should also check Firefox and Safari), therefore we can make $subdocument work properly.

I am also interested in "Sec-Fetch-Mode" - it gives us the ability to distinguish main_frame and decide whether we don't need to use an intermediate blocking page for this case.

Question: what about xmlhttprequest? Can we detect it properly now?