Lint the webroot/rsrc/externals/javelin directory

Summary: Fixes T6953. As discussed, this directory should be linted.

Test Plan: Ran arc lint on a file in webroot/rsrc/externals/javelin and saw linter errors.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6953

Differential Revision: https://secure.phabricator.com/D11375