[xdg_directories] added full example in xdg_directories

[AdamMusa]

This package does not have an example to show people how to use it so I created a complete project with all methods available on this package .
Here is the screenshot of project

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the [CLA].
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy], or this PR is [exempt from version changes].
• I updated CHANGELOG.md to add a description of the change, [following repository CHANGELOG style].
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[stuartmorgan]

Thanks for the contribution!

[x] All existing and new tests are passing.

A significant number of CI tests are failing, including the new widget test, so I'm assuming this is still a work in progress. I'll set this to Draft accordingly; please mark as ready for review once the issues are resolved.

[stuartmorgan]

Closing as obsolete; another PR to add this example has now landed (#4554).