Change syntax highlighting for custom phabricator dot configs

Test Plan:
Created new paste with title '.arcconfig' without choosing a language; observed that the paste gets highlighted as JSON.

JSON mode:
{F4901762}

Javascript mode:
{F4901763}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T11667

Differential Revision: https://secure.phabricator.com/D17682