Exempt some more puncutation characters from diff smoothing in prose diffs

Summary:
Ref T7643. We intentionally smooth diffs, because an edit which changes "The cat ran past." to "The boar ran past." is easier to read if it looks like this:

The (- cat)(+ boar) ran past.

...than if it looks like this:

The (- c)(+ bo)a(-t)(+r) ran past.

However, this rule currently gives us slightly-too-large word diffs for "words" that contain some punctuation symbols, like ] and ). Include these as layout symbols to generate slightly better diffs.

Test Plan: Unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7643

Differential Revision: https://secure.phabricator.com/D16853