feat(popup): Explicilty sets rikaikun to horizontal writing mode

Avoids inheriting veritcal text styles from host page when they're overly broad.

Long term, shadow dom should help, and we should also make a popup position that works well for vertical text.

Fixes #613

extension/css/popup.css

@@ -88,6 +88,7 @@
   padding: 4px;
   position: absolute;
   top: 5px;
+  writing-mode: horizontal-tb;
   z-index: 99992;
 }