Syntax highlight .arclint files as JSON.

Summary: After T2039, it makes sense to syntax highlight .arclint files as JSON.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9305