Add a linter rule for nested namespaces

Summary: Namespace declarations cannot be nested, see https://3v4l.org/kRUNj.

Test Plan: Added unit tests

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14513