R1:8b7e99f68c97

Introduce ModularTransactionType::isRenderingTargetExternal

Summary: This is just some housekeeping - see note in D16287. Basically, "isTextMode" doesn't convey enough information.

Test Plan: `git grep isTextMode | grep -v Remarkup`, and visit all callsites; There are 4 of them left.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17063