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