Pull request: 5258-good-old-filters

Merge in DNS/adguard-home from 5258-good-old-filters to master

Updates #5258.

Squashed commit of the following:

commit 8555e685a104713e552f017de63281749f41b6b2
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Dec 20 16:07:52 2022 +0400

    filtering: imp tests, docs

commit 2ecfc18fc69850a06461620a24527158603cd7b8
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Dec 20 11:00:59 2022 +0400

    filtering: fix docs

commit 1ea8d45a85f3fb6794b44134e8fdcbe2044d2199
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Dec 19 23:19:37 2022 +0400

    filtering: imp naming, docs

commit c52a3bba48738c002111c234fb4c312380e49cfc
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Dec 19 23:13:37 2022 +0400

    filtering: imp logic

commit 3ad4276ace40f05db47b49fb033d1b0fa208ec4e
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Dec 19 17:49:15 2022 +0400

    filtering: imp docs

commit 1bc3cc443bc8ec988532effaaf5f50474a1a69ab
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Dec 19 17:45:37 2022 +0400

    filtering: imp more

commit 7908339a0c9fcc29e8fe12b6c5d8c14bbfa51364
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Dec 19 16:57:42 2022 +0400

    filtering: imp code

commit 21bbd18b4ded83f354210ac32010d8fd1073452f
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Dec 19 12:11:21 2022 +0400

    filtering: imp src reading

internal/filtering/filtering.go

@@ -190,6 +190,8 @@ type DNSFilter struct {
 
 	// filterTitleRegexp is the regular expression to retrieve a name of a
 	// filter list.
+	//
+	// TODO(e.burkov):  Don't use regexp for such a simple text processing task.
 	filterTitleRegexp *regexp.Regexp
 
 	hostCheckers []hostChecker