feat: Add jid_is_valid_user_jid() to check for valid user JIDs

This one will need a localpart as well.
It should be used when we want to message a user for example.
While jid_is_valid() also returns TRUE for domain/server JIDs.

tests/unittests/xmpp/test_jid.h

@@ -30,6 +30,9 @@ void jid_create__returns__null_from_invalid_localpart_chars(void** state);
 void jid_create__returns__null_from_empty_parts(void** state);
 void jid_create__returns__null_from_multiple_at_in_bare(void** state);
 void jid_create__returns__correct_parts_with_at_in_resource_only(void** state);
+void jid_is_valid_user_jid__is__true_for_valid_user_jid(void** state);
+void jid_is_valid_user_jid__is__false_for_domain_jid(void** state);
+void jid_is_valid_user_jid__is__false_for_invalid_jid(void** state);
 void jid_is_valid__is__true_for_valid_jid(void** state);
 void jid_is_valid__is__false_for_invalid_jid(void** state);
 void jid_is_valid__is__false_for_null(void** state);