Add "!#if dns" and "!#if !dns" for AdGuard DNS and AdGuard for Windows/Android's DNS filtering

I remember thinking about something like this a few times earlier this year, but kept forgetting it each time; until I was reminded about it again tonight on Slack.

Essentially, `!#if dns` would be:
True on: AdGuard Home, AdGuard for Windows/Mac/Android's *DNS Filtering*, AdGuard DNS
False on: AdGuard Browser Extension, AdGuard for Windows/Mac/Android's ordinary filtering, uBlock Origin, AdGuard for Safari (all versions), AdGuard Content Blocker

I've forgot which scenarios where I personally would've needed it, but it's perfectly probable for a site to need a domain block/allow in AdGuard Home, while the entry would've simultaneously been way too broad for all other mainstream adblockers.