LibWeb: Allow CSS floating objects to flow across multiple lines

If we run out of horizontal space when placing floating objects,
we now perform a "break" and continue with a new line of floats
below the bottommost margin edge of the current line.

This is definitely not 100% to-spec yet, but a huge improvement
on ACID1 already. :^)

Userland/Libraries/LibWeb/Layout/BlockFormattingContext.h

@@ -56,8 +56,14 @@ private:
 
     void apply_transformations_to_children(Box&);
 
+    enum class FloatSide {
+        Left,
+        Right,
+    };
+
     struct FloatSideData {
         Vector<Box&> boxes;
+        float y_offset { 0 };
     };
 
     FloatSideData m_left_floats;