LibWeb: Implement "out-of-flow" property of Layout Box

In some situations, a layout box should not participate in the standard
layout process, for example when set to `position: absolute`.

Userland/Libraries/LibWeb/Layout/FlexFormattingContext.h

@@ -15,6 +15,8 @@ public:
     FlexFormattingContext(Box& containing_block, FormattingContext* parent);
     ~FlexFormattingContext();
 
+    virtual bool inhibits_floating() const override { return true; }
+
     virtual void run(Box&, LayoutMode) override;
 };