LibWeb: Fix building of areas spanning multiple rows in GFC

Rewrites the grid area building to accurately identify areas that span
multiple rows. Also now we can recognize invalid areas but do not
handle them yet.

Userland/Libraries/LibWeb/Layout/GridFormattingContext.h

@@ -142,6 +142,7 @@ private:
         size_t row_end { 1 };
         size_t column_start { 0 };
         size_t column_end { 1 };
+        bool invalid { false }; /* FIXME: Ignore ignore invalid areas during layout */
     };
 
     HashMap<String, GridArea> m_grid_areas;