Allow prose and code diffs to use different smoothing

Summary:
Ref T7643. The prefix/suffix smoothing affected code diffs, and I think it's worse for them overall.

Generally, code has fewer real "words" than prose does. For example, if ($x) { or $x = ($y && $z); are not "words", and it's correct to highlight very limited parts of them.

Add separate "full" and "internal" smoothing modes for prose and code, respectively. I'll follow up by adjusting arcanist and phabricator.

Test Plan: arc unit --everything

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7643

Differential Revision: https://secure.phabricator.com/D16073