Add a rule to the XHPAST linter for detecting `elseif` usage.

Summary: This rule is adapted from [[https://github.com/squizlabs/PHP_CodeSniffer/blob/master/CodeSniffer/Standards/Squiz/Sniffs/ControlStructures/ElseIfDeclarationSniff.php | PHP_CodeSniffer]] and is used to enforce the consistent use of `else if`.

Test Plan: Wrote and executed unit tests.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin, neal

Differential Revision: https://secure.phabricator.com/D9193