Add a rule to the XHPAST linter to check for whitespace before a semicolon.

Summary: This rule is based on a rule from [[https://github.com/squizlabs/PHP_CodeSniffer/blob/master/CodeSniffer/Standards/Squiz/Sniffs/WhiteSpace/SemicolonSpacingSniff.php | PHP_CodeSniffer]].

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/D9195