LibWeb: Implement minmax()

On the basis of the values passed to the minmax functions, do different
actions given different min and max grid track sizes.

Userland/Libraries/LibWeb/Layout/GridFormattingContext.h

@@ -31,6 +31,8 @@ private:
         CSS::GridTrackSize max_track_sizing_function;
         float base_size { 0 };
         float growth_limit { 0 };
+        float space_to_distribute { 0 };
+        float planned_increase { 0 };
     };
 
     Vector<GridTrack> m_grid_rows;