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/Utilities/pro.cpp

@@ -343,7 +343,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
                 if (output_name.is_empty() || output_name == "/") {
                     int i = -1;
                     do {
-                        output_name = url.host();
+                        output_name = url.serialized_host().release_value_but_fixme_should_propagate_errors().to_deprecated_string();
                         if (i > -1)
                             output_name = DeprecatedString::formatted("{}.{}", output_name, i);
                         ++i;