Do not flip unknown properties starting with "left" or "right"

A property "xxleft" was already ignored, but "leftxx" was still
being matched as the pattern only enforced non-letter before the
keyword, not after.

Inspired by a patch from Tim Landscheidt to the PHP-port of
CSSJanus that ships with MediaWiki:
* https://github.com/wikimedia/mediawiki-core/blob/1.23.3/includes/libs/CSSJanus.php
* https://github.com/wikimedia/mediawiki-core/commit/ce034ac430