Add private field to package manifest

[nezuo]

Closes #9

[magnalite]

This looks good although it's important this functionality always works after we add it. Could you add a test to ensure we don't regress?

The test should be in tests/integration/publish.rs and you can look at check_prompts_auth() to see what it might look like! You'll also want to make a new project in test-projects with a toml containing private set to true.

[ThatTimothy]

Just pulled this locally and tested it, everything looks perfectly fine. The test also fails if I remove the bail segment or cause the package to publish anyways, so it looks good to me!

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@magnalite Is there any update on this? Just asking because it's been 6 months, and this seems like a ready-to-release feature.