No representation for usual ZERO-WIDTH chars

ZWNJ is so common in Persian texts, and its unicode bi-direction class in neutral.
But the representation chunk contains latin characters and are so left-to-right.
So, when ZWNJ is replaced by the representation chunk, rtl texts are rendered incorrectly.

ZWJ and ZWSP are from the same class, but less common at least in Persian texts.

(Related to https://github.com/textmate/textmate/pull/1217)