Add unicode syntax for banana brackets

Summary:
Add "⦇" and "⦈" as unicode alternatives for "(|" and "|)" respectively.

This must be implemented differently than other unicode additions
because ⦇" and "⦈" are interpretted as a $unigraphic rather than
a $unisymbol.

Test Plan: validate

Reviewers: goldfire, bgamari, austin

Reviewed By: bgamari, austin

Subscribers: thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D2012

GHC Trac Issues: #10162