Add Arcanist support for highlighting indent change intraline diffs

Summary:
Ref T13161. See D20181. This allows the intraline highlighter to accept new ">" and "<" spans and apply a different style for them.

The input pattern is list<segment>. Each segment is pair<wild kind, int byte_length>, i.e. wrap the next byte_length bytes in a span of kind kind.

Before this change, the possible kinds of segements are 0 (no intraline diff, do not highlight) or 1 (intraline diff, highlight in bright color).

D20181 adds < (depth decreased) and > (depth increased). These are like 1, but add a different class so the UI can handle them differently.

Test Plan: See D20181.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13161

Differential Revision: https://secure.phabricator.com/D20182