Slightly improve some prose diffs

Summary:
Ref T7643. There are still some silly cases, but I think this improves behavior a bit:

• At the top level, treat whitespace tokens separately. This significantly improves adding or removing paragraphs.
• Fix a bug with diff construction where prefixes and suffixes could overlap on pure whitespace tokens.

There are still some other cases that we don't do a great job on but I think I need to adjust the approach a bit to really improve them.

Test Plan: Added a unit test with a pargraph addition, made it pass.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7643

Differential Revision: https://secure.phabricator.com/D16820