Disable default syntax highlighting for large files in DocumentEngine

Summary: Ref T13105. See also T7895. When users render very large files as source via DocumentEngine, skip highlighting.

Test Plan: Fiddled with the limit, viewed files, saw highlighting degrade.

Reviewers: mydeveloperday

Reviewed By: mydeveloperday

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19306