Add a linter rule for global variables

Summary: Ref T7409. Global variables are gross and should be avoided like the plague.

Test Plan: Added test cases.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Maniphest Tasks: T7409

Differential Revision: https://secure.phabricator.com/D13882