`domain` regex syntax

uBO recently supports `domain` with a regex, e.g.:

```
@@*$ghide,domain=/img[a-z]{3,5}\.buzz/
||video.twimg.com/ext_tw_video/*/*.m3u8$domain=/^i[a-z]*\.strmrdr[a-z]+\..*/
```

These rules are currently parsed by adblock-rust but are treated as if the regex part is a domain.

This also causes issues for content-blocking syntax conversion, so it should be disallowed there.