Add support for modern flexbox layout

Flexbox model will replace old xul box as the default display model so
this patch adds support for that in a whole bunch of styles.

A lot of style rules are marked as "Fx < 112 compatibility" rules and
those can be removed when 112 hits release.

chrome/textual_context_navigation.css

@@ -9,6 +9,7 @@ See the above repository for updates as well as full license text. */
   padding-block: 0.5em;
   padding-inline-start: 1em;
   -moz-box-pack: start !important;
+  justify-content: start !important;
   width: auto !important;
 }
 #context-navigation > menuitem[_moz-menuactive="true"]:not([disabled]){ background-color: -moz-menuhover !important; }