Default to ~/.config/gmailctl for newly created config dirs.

[lgarron]

This is a minimal fix for #144

It is a revision of #393 and uses a library for the internal XDG path calculation per feedback there.

[mbrt]

Suggested change

	Configuration and credentials are in a shared directory, either in a default orq
	custom location.
	- `~/.config/gmailctl` — This is the current default location.
	- `~/.gmailctl` — This was the previous default location. `gmailctl` will continue
	to use this location if it exists on your device.
	- Custom location: use the `--config` argument.
	Configuration and credentials are in either:
	- `<XDG_BASE_DIR>/gmailctl` (using the [XDG base directory spec](https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html), commonly `~/.config/gmailctl` on Linux.
	- `~/.gmailctl`: The previous default location. If the directory exists `gmailctl` will continue to use it for backward compatibility
	- Custom location: use the `--config` argument.

[lgarron]

Yeah, I wasn't sure how verbose to be. Anyhow, it's also the same on macOS (where I use gmailctl).

[mbrt]

Ah alright, then you can add "and MacOS" as well.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.56%. Comparing base (1316284) to head (2f01d10).

Files with missing lines	Patch %	Lines
cmd/gmailctl/cmd/root_cmd.go	0.00%	6 Missing ⚠️

❌ Your patch check has failed because the patch coverage (0.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #455      +/-   ##
==========================================
- Coverage   37.60%   37.56%   -0.05%     
==========================================
  Files          54       54              
  Lines        3670     3674       +4     
==========================================
  Hits         1380     1380              
- Misses       2199     2203       +4     
  Partials       91       91              

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