AK+Everywhere: Change URL::path() to serialize_path()

This now defaults to serializing the path with percent decoded segments
(which is what all callers expect), but has an option not to. This fixes
`file://` URLs with spaces in their paths.

The name has been changed to serialize_path() path to make it more clear
that this method will generate a new string each call (except for the
cannot_be_a_base_url() case). A few callers have then been updated to
avoid repeatedly calling this function.

Userland/Utilities/pro.cpp

@@ -324,7 +324,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
                 }
 
                 if (output_name.is_empty())
-                    output_name = url.path();
+                    output_name = url.serialize_path();
 
                 LexicalPath path { output_name };
                 output_name = path.basename();