Add a linter rule for list assignments

Summary: Add a linter rule to prevent trailing commas in a list assignment. list($x, $y,) is equivalent to list($x, $y), but the latter is cleaner and should be preferred.

Test Plan: Added test cases.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D13870