AK: Port URL username/password from DeprecatedString to String

And for cases that just need to check whether the password/username is
empty, add a raw_{password,username} helper to avoid any allocation.

Userland/Libraries/LibWeb/HTML/NavigableContainer.cpp

@@ -215,8 +215,8 @@ static bool url_matches_about_blank(AK::URL const& url)
     // A URL matches about:blank if its scheme is "about", its path contains a single string "blank", its username and password are the empty string, and its host is null.
     return url.scheme() == "about"sv
         && url.serialize_path() == "blank"sv
-        && url.username().is_empty()
-        && url.password().is_empty()
+        && url.raw_username().is_empty()
+        && url.raw_password().is_empty()
         && url.host().has<Empty>();
 }