RLM-add-ZWNJ by Jonathan Kew

Adds a Zero-Width Non-Joiner after any Right-to-Left Mark characters in the content. This can fix the rendering of some mis-spelled Persian content, for example. See discussion in <a rel="nofollow" href="https://prod.outgoing.prod.webservices.mozgcp.net/v1/f26262120f3317832dc4b46ac99d1dd56269c9a2eaacfcb79d150778c3b5bab1/https%3A//bugzilla.mozilla.org/show_bug.cgi%3Fid=1433886">https://bugzilla.mozilla.org/show_bug.cgi?id=1433886</a>.