Fix login when xdg config home directory does not exist

[pporcher]

This fixes an issue where login function was not able to write plash_config.json file if the xdg_config_home directory does not already exist.

Here is a dialog that reproduce the issue in a fresh solveit instance where xdg_config_home directory, aka /app/data/.config is not created yet.

The fix uses fastcore's Path extension write_json that does create parent directories if they do not exist.

[gitnotebooks]

Found 1 changed notebook. Review the changes at https://app.gitnotebooks.com/AnswerDotAI/plash_cli/pull/75

[RensDimmendaal]

Thanks! lgtm, nice use of fastcore's Path.write_json. Tagged @jph00 for review.