Function default_rcfile_path 

pub async fn default_rcfile_path(
    env: &mut Env,
) -> Result<String, DefaultFilePathError>

Finds the path to the default rcfile.

The default path is determined by the value of the ENV environment variable. The value is parsed as a Text and subjected to the initial expansion.

If the variable does not exist or is empty, the result will be an empty string.

If the variable value cannot be parsed or expanded, an error message will be printed to the standard error and an empty string will be returned.

TODO: If the POSIXly correct mode is off, the default path should be ~/.yashrc (or maybe some XDG-compliant path).