Add a linter rule for unnecessary use of the final modifier

Summary: Ref T7409. This was based on rhttps://github.com/squizlabs/PHP_CodeSniffer/blob/master/CodeSniffer/Standards/Generic/Sniffs/CodeAnalysis/UnnecessaryFinalModifierSniff.php.

Test Plan: Added unit tests.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7409

Differential Revision: https://secure.phabricator.com/D12135