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