Add a linter rule for abstract methods within an interface
Summary:
interfaces cannot contain abstract methods. This construct will cause a PHP fatal error:
Access type for interface method SomeInterface::someMethod() must be omitted
Test Plan: Added test cases.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: Korvin
Differential Revision: https://secure.phabricator.com/D14562