Function hm::config::ensure_config_dir

pub fn ensure_config_dir() -> Result<PathBuf, &'static str>

Make sure $XDG_CONFIG_DIR exists. On Linux and similar this is /home/<username>/.config; macOS /Users/<username>/.config.

Assuming it does exist or we can create it, stuff config.toml on the end of it and return Ok(my_path_buf/config.toml).

This is safe because ensure_config_dir() is called only after we already know the user didn't specify a config.toml path themselves. We must check our default expected location for it.