Add support for DNS filtering syntax validation

I suggest adding support for linting DNS filtering rules as well:
https://adguard-dns.io/kb/general/dns-filtering-syntax/

We'll need to differentiate DNS lists from traditional lists and it seems that the "user agent" string can be used for that. For instance, `[AdGuard DNS]`.

Start with the AdGuard DNS filtering syntax reference linked in the issue and review how the linter currently distinguishes traditional lists from DNS lists. Define validation coverage for DNS rules and recognition via the `[AdGuard DNS]` user-agent string; done means DNS lists are identified and their syntax is linted without breaking traditional-list handling.

Escrita pelo modelo de indexação a partir do texto da issue.