Resolve environment variable values

[invasy]

Fix #421: add support for embedded variables.

• Substitute environment variable values mentioned as $VAR or ${VAR}.
• Detect self-referenced variables (infinite recursion).

[coveralls]

Coverage: 91.956% (+0.3%) from 91.635% when pulling 5897705 on invasy:main into 8874288 on joke2k:main.

[sergeyklay]

Thank you for your contribution to django-environ. Your code looks good and is in line with the project's philosophy. I am ready to accept your changes, however, there are a few things that need to be done before I can do so:

Add documentation for the new feature: This is a very important aspect for the users of this project. Please ensure that you've added detailed information on how to use the new feature, including examples and possible use cases.

Update the changelog: This will help the community stay informed about the changes happening in the project. Please add information about your new feature and how it affects the current functionality of the project.

After completing these two items, I'll be more than happy to accept your Pull Request. If you have any questions or need assistance, please feel free to reach out.

Finally, send this PR to develop branch, not main.

Thank you for your collaboration and efforts in improving django-environ!

[invasy]

Closed in favor of pull request #468 with fixed issues.