If a user passes both `config` and `extraConfig` to the `yabai` serivce,
the generated `yabairc` file is invalid. This is because we do not add a
newline separator when we concatenate the config string generated by
`toYabaiConfig cfg.config` with `cfg.extraConfig`.
This PR prepends a newline to `cfg.extraConfig` if it is non-empty so
that the resulting `yabairc` config is valid.