Generate syntax highlighting CSS from a reusable map

Summary:
Ref T9790. This prepares the syntax color rules to be reused in mail.

This goes about halfway toward T5701 by sort-of supporting different styles but not really.

Test Plan:

• Ran bin/celerity syntax to regenerate syntax map.
• Viewed some highlighted code, didn't see any differences.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9790

Differential Revision: https://secure.phabricator.com/D15846