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