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