fix style-sanitizing regex

in commit 8ae4ba8e, the regex was modified to accept negative
values (e.g., -1.0em), making the \w-\w clause redundant and
causing extremely slow matching in certain pathological cases.

test plan:
 - import the Angel package attached to the ticket
 - it should complete in a reasonable time (a few minutes,
   probably) and definitely should not max out your CPU
   for three hours while making no visible progress

fixes CNVS-10820

Change-Id: I582386c25d4d99f362ce2411b0c387bc958a71c5
Reviewed-on: https://gerrit.instructure.com/30074
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>