The keys that insert by with_list_parse_key() function should to_lowercase

I'm trying to prase the key in the environment like

prefix_keylist : value1_value2
prefix_key: value

I define the source like

            config::Environment::with_prefix("PREFIX")
                .with_list_parse_key("KEYLIST")
                .separator("_")
                .list_separator("_")
                .try_parsing(true),

When try_parsing the key value, I find I can't get the correct result, so I try to debug and then find the list_parse_key not covert to lowercase as in the below image.

Maybe we should convert the list parse key to lowercase when insert the keys

Start by tracing with_list_parse_key() through the environment key insertion and try-parsing path, using the configuration shown in the issue as the reproducer. Check where list_parse_key is normalized and verify that the prefix_keylist and prefix_key example produces the expected parsed values after the change.