[Linting rules] Add a rule to warn not to use .Private namespaces

[allenjzhang]

Clear and concise description of the problem

A few specs are including .Private from either Azure.Core or ARM libraries. We should warn about not using these.

Checklist

• Follow our Code of Conduct
• Check that this issue is about the Azure libraries for typespec. For feature request in the typespec language or core libraries file it in the TypeSpec repo
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.