Add a linter rule for array separators
Summary:
Adds a rule to ArcanistXHPASTLinter which ensures that:
- Single-lined arrays do not contain an unnecessary trailing comma separator after the final array value.
- Multi-lined arrays do contain a trailing comma seperator after the final value value.
Depends on D10534.
Test Plan: Wrote and executed unit tests.
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D10535