[Redesign] Add color to diff Table of Contents

Summary: Ref T8099. I think these colors make sense, but if any seem wrong, lmk. Colors the type of file change.

Test Plan: Test a few diffs locally, read carefully, ask epriestley.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13502