Add MulticolFlow and use it for multicol elements.

It currently "inherits" from BlockFlow and does not override anything.

components/style/properties.mako.rs

@@ -5194,6 +5194,12 @@ impl ComputedValues {
         ))
     }
 
+    #[inline]
+    pub fn is_multicol(&self) -> bool {
+        let style = self.get_column();
+        style.column_count.is_some() || style.column_width.is_some()
+    }
+
     #[inline]
     pub fn get_font_arc(&self) -> Arc<style_structs::Font> {
         self.font.clone()