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