Auto merge of #13978 - Haggus:master, r=emilio

Implement column-fill

Implemented column-fill from #13764

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components/style/properties/longhand/column.mako.rs

@@ -251,3 +251,6 @@
         }
     }
 </%helpers:longhand>
+
+${helpers.single_keyword("column-fill", "auto balance",
+                         products="gecko", animatable=False)}