LibWeb: Support parsing column-span

Author: https://github.com/samfry13
Commit: 23b4367c9c
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/1156
Reviewed-by: https://github.com/AtkinsSJ ✅

Userland/Libraries/LibWeb/CSS/StyleProperties.h

@@ -75,6 +75,7 @@ public:
     CSS::Display display() const;
     Optional<CSS::Float> float_() const;
     Optional<CSS::Clear> clear() const;
+    Optional<CSS::ColumnSpan> column_span() const;
     struct ContentDataAndQuoteNestingLevel {
         CSS::ContentData content_data;
         u32 final_quote_nesting_level { 0 };