AK: Serialize URL hosts with 'concept-host-serializer'

In order to follow spec text to achieve this, we need to change the
underlying representation of a host in AK::URL to deserialized format.
Before this, we were parsing the host and then immediately serializing
it again.

Making that change resulted in a whole bunch of fallout.

After this change, callers can access the serialized data through
this concept-host-serializer. The functional end result of this
change is that IPv6 hosts are now correctly serialized to be
surrounded with '[' and ']'.

Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp

@@ -44,7 +44,7 @@ static bool url_matches_about_blank(AK::URL const& url)
         && url.serialize_path() == "blank"sv
         && url.username().is_empty()
         && url.password().is_empty()
-        && url.host().is_null();
+        && url.host().has<Empty>();
 }
 
 // FIXME: This is an outdated older version of "determining the origin" and should be removed.